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Diamonds I

Having greatly enjoyed writing my last blogs on garnets as gemstones and mineral specimens, topics different from my long-time interests in the ore-forming minerals, I am starting a series of blogs in the same vein on the precious gems: diamonds, emeralds, rubies, and sapphires. 

In the first three blogs the topic is diamonds.  In this first blog, I will describe some famous diamonds and their histories, and follow up with a gallery of diamond jewelry and diamond-decorated art objects from over the years.  In the second blog I will briefly discuss the crystal structure of diamonds, which underlies their physical and optical properties, including their brilliance and colors, and how diamonds are formed deep within the earth’s mantle and then ejected to the surface.  In the third blog I’ll end the series on diamonds with a photo gallery of natural uncut diamond specimens, emphasizing varieties of crystal forms and colors. 

THE BEAUTY AND ALLURE OF DIAMONDS

Not only do diamond gemstones display a remarkable sparkling brilliance, their colored varieties, perhaps unexpectedly, span the spectrum of the rainbow (Figure 1).  These properties underlie their being esteemed over the years by collectors and wearers of these gems.  Diamond’s beauty and early rarity due to limited sources, along with their aura of great wealth and power, underlay their being worn as crown jewels, being set as the gemstones of royals and glitterati, and being set in jewelry, which are also art objects.  Some famous gemstones and other art objects are described below.

Figure 1.  Colors of diamonds. 

The Koh-i-noor Diamond [Ref 1]

The rough stone was mined in the alluvial sands in the Golconda, the ancient diamond-mining region of India [Ref 2].  Its name is Persian for “Mountain of light”, attesting to its brilliance.  Prior to entering the written record, it was shown in 1628 as being set at the top of the gem-laden throne of the Mughal ruler Shah Jahan (Figure 1) [Ref 3]

Figure 2.  Peacock Throne of the Mughal ruler Shah Jahan [Ref 3].

Following its possession by Mughals, it was taken from India in 1739 by the Persian ruler Nadar Shah among the spoils of war.  The gem remained in what is now Afghanistan until, after decades of fighting, it was returned to India by the Sikh ruler Ranjit Singh in 1813.  After Singh’s death in 1839 the gem passed from ruler to ruler before being possessed by a boy, Duleep Singh and his mother, Rani Jindin.  Following her imprisonment by the British, who, as the British East India Company, expanded into central India, ten-year old Duleep was forced to give away the Koh-i-noor and his claim to sovereignty.  From there the diamond came into the possession of Queen Victoria after it was recut to enhance its brilliance (Figure 3) [Ref 4].  It was set in a brooch, (Figure 4) [Ref 5], and became part of the British Crown Jewels [Ref 6].  After resetting, the diamond now resides at the front of the crown of the Queen Mother, mother of Queen Elizabeth II (Figure 5). 

Figure 3.  The Koh-i-noor diamond before and after re-cutting [Ref 4].
Figure 4.  Queen Victoria wearing the brooch with the recut Koh-i-noor diamond [Ref 5].
Figure 5.  Queen mother’s crown with the round brilliant recut Koh-i-noor Diamond [Ref 6].

The Cullinan Diamond [Ref 7]

More recently found than the Koh-i-noor, the Cullinan diamond (Figure 6) was found in 1905 at the, (then called), Premier Mine in Johannesburg, South Africa.  At 3106 carets (1.371 lbs.) it was the largest rough diamond ever found, until the discovery of a 1109 caret diamond in Botswana.  Cutting it provided seven gemstones (Figure 7) including the Cullinan I and Cullinan II diamonds.  These were set respectively in the Sovereign’s Scepter (Figure 8) and in the Imperial State Crown (Figure 9), both among the British Crown Jewels.

Figure 6.  The rough Cullinan Diamond as discovered [Ref 7]. 
Figure 7.  The major gems from the Cullinan diamond [Ref 8].
Figure 8.  The Sovereign’s Scepter of England with the Cullinan I Diamond [Ref 9].
Figure 9.  The Imperial State Crown of England with the Cullinan II diamond, the St. Edward’s Sapphire and the Black Prince’s “Ruby” (actually a red spinel) are also set on the crown [Ref 10].


The Hope Diamond [Ref 11]

Reportedly cursed, after being stolen from the eye of a holy Hindu statue, the original 115.16 ct stone was likely purchased by Jean-Baptiste Tavener, a diamond merchant, from the Kollur Mine in Golcanda, India and sold in 1668 to King Louis XIV of France, and became known as the “French Blue”.  It was recut to a 67 ct gem by the court jeweler [Ref 11].  During the French Revolution the French Blue was stolen.  Later the stone changed hands and went to Henry Phillip Hope. It is now famously known as the Hope Diamond.  After a series of purchases, it was donated to the Smithsonian Institution in 1958 where it resides today.  After its acquisition by the museum the weight of the stone was determined to be 45.52 carats. Its intense blue color is seen in Figure 10. 

Figure 10.  The Hope Diamond [Ref 11].

The Orlov Diamond [Ref 12].

The rough diamond ancestor of the Orlov Diamond was mined in the Kolliur Mine in the Golcanda region of India [Ref 12] and weighed 787 carats. Following it’s residence as an eye of a deity, in a temple on the island of Srirangam, in South India, it made its way to Amsterdam, and around 1768 was sold to Count Grigoryevich Orlov, a Russian nobleman and officer who helped Catherine the Great overthrow Peter III and gain the throne of Russia.  Catherine had the diamond mounted on the Imperial Scepter (Figure 11).  The diamond has a non-traditional shape, resembling half of a bird’s egg.  It is a white diamond [Ref 13] with a bluish-green tint and weighs 189 cts.

Figure 11.  The Olov Diamond atop the Russian Imperial Scepter [Ref 13].

The Tiffany Yellow Diamond [Ref 14]

This rough diamond, weighing 287 cts, is the largest yellow diamond ever mined. Purchased by Charles Lewis Tiffany, founder of Tiffany & Co., in 1877.  It was fashioned into a cushion-cut brilliant, weighing 128 cts.  The gemstone was fashioned into a diamond necklace (Figure 12) and worn by Audrey Hepburn in promotional photos for the film Breakfast at Tiffanys.  Later the gemstone was mounted to create the whimsical “Bird on a Rock” brooch (Figure 13).  The stone was reset into a diamond and platinum necklace in 2012 to commemorate the 175thanniversary of Tiffany & Co.

Figure 12.  Necklace with the Tiffany Yellow Diamond as worn by Audrey Hepburn [Ref 16].

Figure 13.  Bird on a Rock brooch with Tiffany Yellow Diamond [Ref 15]
 

Gallery of Diamond Jewelry as Art Objects

Figure 14.  Mukut (Crown) head ornament of Lord Vishnu with diamonds and other precious stones (1300-1500) [Ref 17].
Figure 15.  Mughal turban ornament with diamonds, rubies, emeralds [Ref 18].
Figure 16.  (Inscribed Shah diamond) [Ref 19]
Figure 17.  Indian Prince Diamond necklace [Ref 20].  The triangular gems are worked twinned crystals (Macles).  See gallery of specimens in the Diamonds III blog.
Figure 18.  Nephrite jade box with diamonds, rubies, and emeralds [Ref 21].
Figure 19.  Art Deco diamond, emerald, and platinum bracelet, ca. 1915 [Ref 22].
Figure 20.  Art Deco Diamond, Coral, Onyx and Platinum Ring [Ref 23].

Fluorite Art Works & Jewelry

Artworks and jewelry have been created from fluorite since ancient times, such as the carved fluorite statues discovered in Crittenden County, Kentucky and shown in Figure 1 [Ref 1]. The statues reflect the Middle Mississippian Culture extending approximately 800 Ad to 1600 AD. The second statue from the left, known as the Tolu Statue is dated to 1150-1200 AD. The fluorite for these carvings undoubtedly came from Southern Illinois and the adjacent region of Kentucky, which were centers of fluorite mining from the early 1800s until 1995.

Figure 1. Carved fluorite statues dating from the Middle Mississippian Culture era [Ref 1].


Another example of art work in fluorite is the beautiful antique carved statue of Quan Lin, the Chinese goddess of mercy and compassion displayed in Figure 2 [Ref 2, Ref 3].

Figure 2. Fluorite statue of Quan Lin the Chinese Goddess of mercy and compassion [Ref 2, Ref 3].

Crafting art works and gemstones of fluorite demanded the artisan’s appreciation of its low hardness of 4 on the Moh Hardness Scale, and its perfect, (and easy) octahedral cleavage [Ref 4], as well as their great artistic skill. For example, faceting the fluorite gemstones, shown in Figure 3, required they determine the orientation of the fluorite crystal to minimize stress along the eight cleavage planes in the crystal, while cutting the stone. Larger artworks were carved or formed using polycrystalline fluorite so that the interlocking of the multiple crystal faces prevented propagation of cleavage while working the art object. Careful examination of Figures 1 and 2, and the artworks in the following figures, will disclose their polycrystallinity.

Figure 3. Faceted fluorite gemstones [Ref 5].

Cabochons often are formed of banded fluorite for their beautiful display of multiple colors as shown in Figure 4.

Figure 4. Cabochons of banded fluorite [Ref 6]. The interlocking of micro-crystals in the bands and at their interfaces prevented cleavage of the stone during cutting.

GALLERY OF JEWELRY AND ART WORKS  –  In the following gallery diverse, beautiful examples of artworks and jewelry representing the creative skills of the artisans are presented in Figures 5-16.

Ref 1. http://www.lithiccastinglab.com/gallery-pages/2011junetolustatuepage1.htm

Ref 2. https://www.rubylane.com/item/1061514-0015-7084/Antique-Quan-Yin-Carved-Fluorite-Stand

Ref 3. https://en.wikipedia.org/wiki/Guanyin

Ref 4. https://www.mindat.org/min-1576.html

Ref 5. https://www.gemsociety.org/article/fluorite-jewelry-gemstone-information

Ref 6. https://www.healingcrystals.com/Cabochons_-_Rainbow_Fluorite_Cabochon__India_.htm

Ref 7. https://www.gemsociety.org/article/fluorite-jewelry-gemstone-information/

Ref 8. https://en.wikipedia.org/wiki/Blue_John_(mineral)

Ref 9. https://www.ebay.co.uk/itm/ANTIQUE-CHINESE-HARDSTONE-BLUE-JOHN-SNUFF-BOTTLE-18TH-C-/202054494962

Ref 10. https://www.pinterest.com/pin/536913586810773819/?lp=true

Ref 11. https://www.pinterest.ca/pin/158259374378896038/

Ref 12. https://www.gemsociety.org/article/fluorite-jewelry-gemstone-information/

Ref 13. http://www.antiques.com/classified/Asian-Antiques/Asian-Decorative-Arts/Antique-Fluorite-Carving-Of-Grape-Cluster

Ref 14. https://beadage.net/gemstones/fluorite/

Ref 15. http://www.incense-burner.com/index.php?st=c&mat=Jade/Agate/Fluorite

Ref 16.https://www.florencejewelshop.com/product/green-silver-fluorite-necklace-of-fluorite-sterling-silver-with-a-silver-magnetic-clasp-lomboks-dream/#iLightbox[product-gallery]/0

Ref 17. https://www.ebay.com.my/b/Toucan-Collectibles/165275/bn_3028968

Ref 18. https://www.alibaba.com/product-detail/Fluorite-Gemstone-Silver-Necklace-925-Sterling_114584988.html

Ref 19.  https://www.pinterest.com/pin/570760952750804859/?lp=true

Fluorite, A Collectors Favorite

The rich, often “electric” rainbow colors of fluorite specimens from world-wide locations, the frequent appearance of different colors within regions, bound with the lattice planes within a crystal, and the sculptural appearances of stepped cubic and octahedral crystals, as shown in Figures 1-5, make fluorite a favorite among collectors. 

In this blog I’ll describe those structural properties of fluorite that are relevant to lapidary practices, and the sources of its’ color, (Figure 8).  I’ll describe how the regions, or zones, of different colors within a fluorite crystal develop. Also, because of their beauty, I am including a gallery of the beautiful fluorite specimens from around the world, that I found as I searched the web.

Figure 1. Stepped color-zoned fluorite crystals with boundaries parallel to the cubic faces of the crystals, Ozark-Mahoning Mione, Cave-in-Rock sub-District, Illlinois-Kentucky Fluorite District, Hardin County, Illinois [Ref 1]
Figure 2. Color-zoned fluorite on quartz with diagonal boundaries between the green and purple regions directed along dodecahedral planes in the fluorite crystals, Kudubis Farm 19, Karibib District, Erongo Region, Namibia [Ref 2].
Figure 3. Color-zoned fluorite with boundaries along diagonal dodecahedral planes within the cubo-octahedral crystal, White Water, Pigeon Rock Mountain, Mourne Mountains, County Down, Northern Ireland, UK [Ref 3].
Figure 4. Stepped fluorite cubic crystal, Spain [Ref 4]
Figure 5. Stepped fluorite cubo-octahedron, Shangbao Mine, Leiyang, County, Hengyang Prefecture, Hunan Province, China [Ref 5].

Properties of Fluorite [Ref 6]

Crystal Structure and Crystallography

The crystal lattice of fluorite exhibits cubic symmetry with the calcium and fluorine ions positioned within the basic unit cell, as shown in Figure 6. The four fluorine ions F-1are positioned centrally in the unit cell with the calcium ions Ca2+distributed at the corners and the center of each face of the unit cell. In this array, each calcium ion has two fluorine ions as neighbors. Imperfections in the lattice underlie the presence of color in fluorite, as described below.

The lattice structure of fluorite results in its crystallizing in the isometric crystal system. Fluorite typically exhibits the basic crystal forms of the cube, octahedron, and rhombic dodecahedron shown in Figures 7 & 8. Fluorite also exhibits ball-like or grape-like botryoidal forms, but only rarely. (Figure 9)

The Mohs hardness of Fluorite is 4 and it exhibits a perfect cleavage on octahedral planes and less-well developed cleavage or parting on dodecahedral planes. A cleavage octahedron is shown in Figure 12 [Ref 14]. The ready cleavage and softness of fluorite requires the lapidarist to minimize the generation of stresses during the cutting and polishing of a stone.

Figure 6. the crystal lattice of fluorite [Ref 6].

Typical Crystal forms of Fluorite

Typical crystal forms are the cube, oxctahedron, and dodecahedron; fluorite also occurs in botryoidal form.

Figure 7. Cubic crystals of fluorite, Las Causses, France [Ref 7].
Figure 8. Fluorite octahedrons on smoky quartz, Argentiere Massif,
Mont Blanc, Chamonix, France [Ref 8]
Figure 9. Cubic fluorite crystal with edges modified by dodecahedral faces, La Cabana, Asturias, Spain [Ref 9].
Figure 10. Botryoidal fluorite with calcite on quartz, Mahodari Quarry, Nasik, State of Maharashtra, India [Ref 10].

Twinning in Fluorite

Fluorite forms both penetration twins [Ref 11] and Spinel Twins [Ref 12] on octahedral planes, as shown in Figures 10 and 11.

Figure 11. Fluorite penetration twin, Lake George, Florissant, Colorado [Ref 13].
Figure12. Fluorite spinel Twin, Erongo, Namibia [Ref 12]. The octahedral face of one crystal is seen on the upper surface of the twin. The other face is hidden by the crystal. The outside trace of the twin plane is irregular [Ref 14].

Cleavage in Fluorite 

Figure 13. Fluorite cleavage octahedron. Cave-in-Rock District, Hardin County, Illinois [Ref 15]. The cleavage is perfect

Colors in Fluorite 

As shown in Figure 6, fluorite can occur in colorless form. Color, as described in Table I, is induced by the presence of an impurity ion, by nanoparticles of calcium metal, which scatter light preferentially according to size, or by inclusions of organic material such as bitumen.

Figure13. Color center in fluorite [Ref 22]. REE = rare earth element
 Ref 22. Page 27, Schematic of photochromic center color center–:

Color Zoning in Fluorite Crystals 

The establishment of discrete geometric zones of color in a fluorite crystal comes from the sequestration of a specific impurity, advancing along a crystallizing plane during sectorial growth [Ref 23].  A sketched example of sectorial growth in a fluorite crystal is shown in Figure 15, which is taken from Figure 4 of [Ref 24]. In the example, growth of octahedral and cubic planes are shown.  In Figure11, showing a fluorite specimen from Namibia, growth is seen to have proceeded along both dodecahedral and cubic planes. Growth proceeded along advancing dodecahedral planes, forming the green diamond-shaped region, and along cubic planes, forming the deep green corner regions.

Figure 14. Schematic of growth of sectors in a fluorite crystal. In generation of dodecahedral and cubic sectors as in Figure 14, growth proceeds towards the four cube corners and faces, forming four-sided pyramids within the crystal [Ref 23]. 
Figure 14. Color-zoned fluorite, Okorusu mine, Namibia [Ref 24].

GALLERY OF FLUORITE SPECIMENS 

In this Gallery I’ve included specimens from France, Morocco, Germany, Spain, and Mexico, which are not as well known to collectors as specimens from England the United States.

Ref 25. http://www.trinityminerals.com/sm/frnfluor.shtml

Ref 26. http://www.webmineral.com/specimens/picshow.php?id=3141&target=Fluorite#.W7KDboUXsws

Ref 27. https://www.pinterest.com/pin/111534528254934024/

Ref 28. https://www.mineral-forum.com/message-board/download.php?id=6836&sid=5476579b13c3f45b45dcbab1a1eafaf8

Ref 29. https://www.pinterest.com/pin/592716000930774044/

Ref 30.  https://www.crystalclassics.co.uk/events/1/43rd-gem-and-mineral-show-aue/

Ref 31. http://www.johnbetts-fineminerals.com/jhbnyc/mineralmuseum/picshow.php?id=24766

Ref 32. https://picclick.com/Fluorite-Asturias-Spain-372413243400.html

Ref 33. https://www.mineratminerals.com/archive/pink-fluorite-2c-la-mina-navidad-2c-mexico-detail

Ref 34. https://www.minfind.com/mineral-520150.html

An Unusual Quartz Crystal

While at the Flagstaff Gem, Mineral & Jewelry show this past August, my fellow Coconino Lapidary Club member and collector friend, Gordon, and I spotted an unusual appearing polished quartz crystal. Upon closer inspection we spotted a geometrical array of fine lines within the crystal which looked to be perpendicular to each other. The appearance of the fine lines seemed similar to the line of incomplete cleavage shown in the octahedral diamond crystal. Mindat, [Ref 1]cites quartz as exhibiting cleavage, as well as fracture, (surprising to me upon discovery, as most sources I’ve seen describe quartz as only exhibiting fracturing), I suggested that the fine lines might be due to cleavage. Gordon purchased this crystal, (shown in Figure 1), with the intention of our investigating it to ascertain if the lines were indeed due to cleavage.

The result of our efforts was a yes. The sharp lines we observed were caused by cleavage along rhombohedral planes, as indicated by our measured and calculated angle of 87.5 degrees between the lines, agreeing well with the literature value of 86 degrees.  How we got here is described below, along with how to go about finding demonstrations of cleavage in quartz along other crystallographic planes and how to obtain values for the compressive strength of quartz from the literature.

Description of Cleavage in Quartz

In its description of quartz, Mindat states that quartz fractures exhibiting a conchoidal, (shell-like), surface [Ref 1] and, with respect to cleavage in quartz, that when “The rhombohedral cleavage {10-11} is most often seen, there are at least six others reported.”. This summary led to a web-search resulting in papers which described cleavage occurring in quartz due to shearing along lattice planes, which is due to stresses induced by very high static pressure [Ref 2].

A Cleavage Pattern Under Static Pressure

Fragments of large, single crystals of quartz, which had been embedded in veins located in the basement rocks in Madagascar during the age of the supercontinent, Gondwana, [Ref 3], were used in the study we examined. Separation of India from Madagascar incurred intense pressures and shearing of the basement rocks. Micrographs of thin sections, prepared from the fragments, disclosed two sets of parallel cleavage lines running essentially perpendicularly to each other, as shown in Figure 2 (Figure 2 of Ref 2). Orientation of the thin section by x-Ray diffraction [Ref 4] allowed assignment of cleavage, in both sets of parallel lines, to be directed along rhombohedral planes.

Are the Essentially Perpendicular Lines in the Crystal of Figure 1 Due to Cleavage? 

Comparison of the array of essentially perpendicular cleavage lines, with the two prominent essentially perpendicular lines on the left side of the quartz crystal, in Figure 1, suggested that these lines and others in the crystal might represent cleavage planes. In order to examine this possibility, the angle between the pair of approximately perpendicular lines on the left side of the crystal, were used to obtain a value for the angle between the nearly perpendicular sides. An estimate for the angle was obtained by measuring the lengths of the sides of the resulting right triangle, constructed by erecting a perpendicular to one side and computing the tangent of the included acute angle. This value allowed computation of the arctangent of the angle, giving the computed value of 87.5 degrees, which agrees well with the value of 86 degrees between the rhombohedral (r-r) faces of the drawn crystal shown in Figure 3, (Figure 3 of Reference 5),  [Ref 5]

Cleavage in Quartz Along Other Crystallographic Planes

Having obtained this result, we next researched the web further, looking for quartz crystals which exhibited evidence of cleavage along other possible planes, as given in Table 1 of Reference 5, as shown in Figure 4. Our search, of some duration, resulted in finding the two quartz crystals shown in Figures 5 and 6.

Examination of the crystal in Figure 5 disclosed two suspected cleavage lines in its lower left region. The lines intersect each other, with one being parallel to the axis of the axis parallel to the unit, or secondary prism, of the crystal and the other line intersecting it at an obtuse angle. Calculation of this angle, from measurements on an enlargement of the photo of the crystal, gave a value of 140.6 degrees which agrees closely with the angle between the unit prism and second order trigonal pyramid, (Table, Figure 5), which is shown in Figure 3 as the angle z-m = 142 degrees. In appearance both lines lack the sharpness of those of rhombohedral cleavage. This is probably due to the camera angle in the photo not being perfectly perpendicular to the cleavage lines.

Examination of Figure 6 disclosed four suspected cleavage lines perpendicular to the axis of the prism and one angled at an angle seen in the rhombohedral cleavage in Figure 1. These lines are due to cleavage along the planes parallel to the basal pinacoid (face) of the crystal (Table, Figure 5).These cleavage lines also lack the sharpness of the rhombohedral cleavage lines in Figure 1, probably due to the camera angle. [Ref 6]

Reported Values of the Compressive Strengths of Quartz and Other Silicate Minerals

We found the following references on the web. All gave results obtained at room temperature ~ 72 degrees F. In two experiments, using uniaxial quasi-static (slowly applied force)directed perpendicularly to the prism face, gave values of 2.55 Gigapascals (369,847 psi) [Ref 7] and 2.74 Gigapascals (981,719 psi) [Ref 8], respectively. In two experiments performed to determine the elastic properties of quartz results were obtained with hydrostatic pressures up to 10 GPa ( 1,740,456 psi) and 12GPa (2,088,547 psi), respectively. [Ref 9]

Figure 1. Quartz, State of Minas Gerais, Brazil [Ref 10].
Figure 2. Cleavage along essentially perpendicular sets of rhombohedral planes in quartz [Ref 2].
Figure 3. Angles between quartz faces (or crystal planes) [Ref 3].
Figure 4. Observed planes of cleavage in a quartz crystal [Ref4].
Figure 5. Polished quartz crystal, State of minas Gerais, Brazil.
Figure 6. Polished quartz crystal, State of Minas Gerais, Brazil

Quartzsite Field Trip

Our January field trip to Quartzsite saw perfect weather in the high 60’s, with Snow Birds filling the open desert with their motor-homes.

Linda and I arrived on the 17th as did John and Beth Duggan. Our rock collecting spree began at Desert Gardens, where just about every rock, mineral and gem can be found, for a price. From slabs to cabs to large medium and small rock, Brazilian amethyst cathedrals and large quartz crystal formations. Next stop was the Q.I.A Pow-Wow where more rock and minerals plus jewelry, tools and rock cutting machinery can be had. More slabs and cabs. Indoor display cases were setup, some from dealers showing their wares and some for competition to be judged.

We four made a day of it, with a final stop at Tyson Wells, a huge flee market type fair. Clothing, garden art, tools, household items, beautiful rugs, walking sticks and essential oils, and this list barely begins to cover what is available. As well as the food courts and lots of people perusing the wares.

On Saturday we met at T-Rocks, in order for any members that made the trip from Flagstaff to meet us, sadly none showed up. Maybe next year, with a little more planning and information about where to find accommodations, we’ll have a better turnout.

All in all it was a fun trip, seeing lots of friends from California and Linda and I helping with the American Lands Access Assoc. desert cleanup on Sunday morning. Then, home. It was an fun and exhausting time, but I’m sure we will be back again next year!

Martin & Linda Dougherty

Calcite II – Jewelry & Art

In this blog, art works and jewelry, dating from 2300-2400 BC to the present, are shown. All were discovered with great fun by googling the web.

Figure 1. Carved calcite (travertine) bowl, 4th -mid 5th Dynasty, 2500-2400 BC [Ref 1].
Figure 2. Carved relief calcite disc, Sumerian
2350-2300 BC [Ref 2].
Figure 3. Carved Egyptian Calcite Canopic Jar with human headcover, 17thDynasty,
1570-1085 BC [Ref 3], [Ref 4].
Figure 4. Gold earrings with calcite, cloisonne’ enamel, earthenware, and glass from tomb of Tutankhamun, 18thDynasty, 1336-1327 BC [Ref 5}
Figure 5. Egyptian carved calcite onyx lion holding a vessel, 17thDynasty, 525-404 BC [Ref 6].
Figure 6. Calcite-alabaster stele, South Arabian Peninsula, circa 3rdto 1stCentury BC [Ref 7].
Figure 7. Calcite statue of Roman emperor Hadrian, body from 4thCentury AD, base, head, and hands of gilt bronze from Italy. [Ref 8].
Figure 8. Offering vessel of calcite onyx in form of an ocelot,
Teotihuacan Culture circa 400-600 AD [Ref 9].
Figure 9. Frankish Disk Brooch, gold sheet with inlays of calcite, garnet, and glass,
lets 7th Century AD [Ref 10].
Figure 10. Brooch of sterling silver with Mexican chromian green calcite [Ref 11]
Figure 11. Carved calcite onyx apple [Ref 12].
Figure 12. Calcite and mahogany obsidian necklace [Ref 13].
Figure 13. Blue calcite egg [Ref 14].
Figure 14. Carved blue calcite bowl, Argentina [Ref 15]. 
Figure 15. Pin/Pendant of cobaltoan calcite druzy and faceted gemstones [Ref 16]

CALCITE JEWELRY AND ARTWORK REFERENCES

Ref 1. https://www.metmuseum.org/art/collection/search/543887?rpp=30&pg=2&gallerynos=103&rndkey=20150416&ao=on&ft=*&pos=55

Ref 2. https://www.penn.museum/collections/object/293415

Ref 3. https://www.dia.org/art/collection/object/canopic-jar-human-head-cover-43587

Ref 4. https://en.wikipedia.org/wiki/Canopic_jar

Ref 5. https://www.pinterest.com/pin/300333868875258132/?lp=true

Ref 6. https://www.brooklynmuseum.org/opencollection/objects/3594

Ref 7. https://www.pinterest.co.uk/pin/386605949244823995/

Ref 8. https://www.mfa.org/collections/object/hadrian-58915

Ref 9. http://www.britishmuseum.org/research/collection_online/collection_object_details.aspx?objectId=479257&partId=1

Ref 10. https://www.alamy.com/disk-brooch-late-7th-century-frankish-gold-sheet-with-copper-alloy-backing-and-inlays-of-garnet-glass-and-calcite-overall-2-14-x-1516-in-57-x-24-cm-metalwork-gold-the-dress-of-frankish-women-generally-consisted-of-a-tunic-cinched-by-a-belt-from-which-hung-an-array-of-pendants-a-wrap-or-cloak-went-over-the-tunic-shoes-and-hosiery-fastened-with-buckles-covered-the-legs-image212485417.html

Ref 11. https://www.ebth.com/items/9279883-mexican-made-sterling-silver-calcite-brooch-and-earrings

Ref 12.https://therockshed.com/polishedrock/pr1116a.jpg

Ref 13. http://www.august-veeck.de/jewellery/necklaces/calcite-obsidian/

Ref 14. https://auction.catawiki.com/kavels/10229271-large-blue-calcite-egg-17-x-11-5-cm-3-33-kg

Ref 15. https://fineart.ha.com/itm/lapidary-art/carvings/blue-calcite-bowlandes-mountainsargentinasouth-america/a/5324-72259.s

Ref 16. https://www.pinterest.com/pin/84653667977299238/

Calcite I

Despite its wide distribution in limestone and as a common gangue mineral in ore deposits, the rainbow of colors and beautiful crystal forms of calcite, from locations around the world, have made it a favorite of collectors. One such favorite is the group of calcite crystals, tipped by hematite, from the Egremont Mine shown in Figure 1. In massive form it has provided lapidists and other artisans a beautiful material for the creation of jewelry and other artworks, such as the sculpture done in Utah calcite, shown in Figure 2.

In this blog I will briefly describe the mechanical properties of calcite, its crystallography, optical properties, and sources of its colors. In Calcite II, I will follow with a gallery of calcite specimens sought by collectors from world-wide locations.

Figure 1. Calcite with a partial hematite coating, Egremont Mine,
Cumberaland, England [Ref 1]
Figure 2. Abstract calcite sculpture “Sundance” carved from Utah calcite [Ref 2].

Mechanical Properties [Ref 3]

The hardness of calcite on the Mohs Scale is 3. It’s brittle because of its perfect cleavage along rhombohedral planes. It parts readily, along twin planes formed by stresses. It can also exhibit conchoidal fracture.

Crystallography [Ref 3, 4]

Calcite crystallizes in the Trigonal System with crystallographic axes, and the often-seen rhombohedral, (Left), and scalenohedral, (Right), forms, shown in Figure 1 [Ref 4]. The system possesses three a1,2,3-axes at 120 degrees with respect to each other in the horizontal plane and the perpendicular c-axis. Other crystal forms can be seen in Ref 3.

Any plane through the trigonal lattice is represented by four numbers (hkmi). These are the Miller Indices which are the reciprocal values of the intercepts respectively on the a1, a2, a3, and c-axis. A family of planes is indicated by the notion {hkmi}.

Calcite forms twinned crystals according to four twin laws [Figure 4 in Ref 5] as shown in Figure 2. The twin forms and the associated family of planes are given in Table I. The angles between the vertical c-axes for each twin form are respectively to the nearest degree of 180, 127, 90, and 53. Calcite specimens exhibiting the twin laws are shown in Figures 5-8.

Among minerals calcite can be considered to be the best one to demonstrate cleavage because of its perfect cleavage along the rhombehedral family of planes {10-11}. The rhombehedral shape of the specimen demonstrating birefringence (double refraction) is evident in Figure 10.

Figure 3. Trigonal Crystallographic System [Ref 4]

Figure 4. Four twin laws of calcite shown with scalenohedral forms [Ref 5]

                                             TABLE 1

                        Twin Form                 Family of Twin planes
                               a                                {0001}
                              b                               {10-11}
                               c                               {01-12}
                               d                               {02-21}
Figure 5. Calcite crystal twinned on the basal (0001) plane,
Elmwood Mine, Carthage County, Tennessee [Ref 6].
Figure 6. Calcite crystal twinned on the rhombahedral plane
(10-11) [Ref 7].
Figure 7. Calcite crystal twinned on the pyramidal plane (01-12) location not stated [Ref 8].
Figure 8. Calcite crystal twinned on the pyramidal plane (02-21)
Brushy Creek mine, Reynolds County, Missouri [Ref 9].

Optical Properties 

Calcite exhibits a range of lusters between vitreous to pearly and its transparency ranges between transparent to translucent. Calcite also exhibits birefringence in its refraction of light within the crystal [Ref 3]. Because the refractive index of calcite varies with the direction of light within a calcite crystal the light entering the crystal is doubly refracted into two different directions, as shown schematically in Figure 3 and demonstrated by the double image of the lines seen in Figure 4.

Figure 9. Double refraction in calcite showing two light paths [Ref 10].
Figure 10. Doubly refracted light in a calcite rhombahedron [Ref 11]

Sources of Color in Calcite 

In its description of the properties of calcite, the mineral reference site mindat lists besides white, a rainbow of colors: yellow, red, orange, blue, green, brown, grey, etc. for calcite.

Calcite, which chemically is calcium carbonate with the formula CaCO3, is colorless when pure, as shown by the crystals on matrix shown in Figure 7 [Ref 12].  The rich colors of calcite specimens arise from different sources, such as substitution of low levels of one of the transition metals its divalent ionic form M+2for the calcium ion Ca+2in the lattice of the crystal, low densities of radiation-induced defects in the lattice of the crystal, or inclusions of a pigmented mineral. 

Colors due to transition metal impurity ions [Ref A]

The colors present with iron and chromium, manganese, cobalt, or chromium present in, respectively ferroan, manganoan, cobaltoan, and chromian calcites are summarized in Table II. The yellow, pink and green colors due to these transmission metals can be seen also in many other minerals as can be seen in a search on the Web. 

                                                                        TABLE II

                             MINERAL                               COLOR
                         Ferroan Calcite                               Yellow
                       Cobaltoan Calcite                                 Pink
                     Manganoan Calcite                                 Pink
                        Chromian Calcite                                Green

Colors arising from radiation damage in the crystal lattice 

The search on the web for what colors of calcite might stem from radiation damage disclosed two references in which blue and amber-colored calcite were associated with radiation damage. Results of the study of References suggested that radiation-induced color centers involving the negative ion CO-3and the presence of stress-induced twInning and lattice disloctions account for the coloring mechanism.

The Mineral Spectroscopy Server of the Divisions of Geological and Planetary Sciences of the California Institute of Technology states that natural radiation induces an amber color in the Calcites from the lead-zinc Tri-State Mining District [Ref B, C]

Colors arising from colored inclusions 

Colorful calcite is also produced by the presence of pigmented inclusions. Clear and transparent crystals of calcite, when included by such strongly colored minerals as malachite, pyrite, hematite, native calcite and others, make highly aesthetic specimens. The inclusions may be dispersed in the cystal (Figure 16) or reside on an included phantom crystal within the specimen crystal (Crystal 17).

Calcite, Bigrigg Mine, Egremont area, West Cumberland Mining District, Cumbria, England [Ref 12]
Figure 12. Ferroan calcite, Boral Limited quarry, Bundoora, City of Whittlesea, Victoria, Australia [Ref 13]
Figure 13. Manganoan calcite, 2ndSovietsky Mine, Dal’ngorsk, Far Eastern Russia [Ref 14]
Figure 14. Cobaltoan calcite with green kolwezite, Kambove Mine, Katanga Copper district, Democratic Republic of Congo [Ref 15].
Figure15. Bladed crystals of chromian calcite, Santa Eulalia District, Chihuahua, Mexico [Ref 16].

Color due to an included pigmented mineral 

Figure 16. Calcite with conichalcite inclusions, Mapimi district, Durango, Mexico [Ref 17].
Figure 17. Calcite crystals containing phantom crystals covered with marcasite, Linwood Mine, Buffalo, Scott County, Iowa [Ref 18].

Dec 2018 Trip to Dobell Ranch

Our December 8th field trip to Dobell Ranch began with a chilly morning meet at Silver Saddle Rd. A scant, lucky, seven members ready to scout the open pits and grounds strewn with multi-colored Arizona Petrified Wood.

Our host, Rhonda Dobell and grandchildren, helped by pointing out some of the better material. It was as if we were shopping in a candy store with so many potential specimens to choose from, you ended up having to be very selective as to what you were bringing home. I think we all found what we wanted and then some. We all would like to have the tree trunks decorating our yards, but decided to be reasonable!

Lunch was prepared for all of us and we had fun with the younger grandkids. Then, when everyone was leaving, there were hugs for all around from the kids.

A very successful field trip that I’m sure we will make a repeat visit to, early next year.

Martin & Linda Dougherty

Gold III – Lost Gold Mines in the Southwest

Tales of the discovery and loss of rich gold mines such as The Lost Dutchman in the Superstition Mountains of Arizona and the El Naranjal lost gold mine in the Sierra Mountains of  Mexico, as popularized by folklorist, J. Frank Dobie, in “Apache Gold and Yaqui Silver” have fascinated many of us. An extensive list of both lost gold and silver mines of the Southwest , each with extensive and entertaining descriptions of their histories, can be found in “Lost Treasure Tales” on the GeoZone Site [Ref 1].

Perhaps the lost mine tale that most of us are aware of, is that of the Lost Dutchman Mine [Ref 2]. One, of 62, hand drawn maps of its supposed location, made available in Reference 3, is shown in Figure 1 and orients it with respect to the prominent geological landmark, Weavers Needle, shown in Figure 2.

Figure 1. Map of the location of the Lot Dutchman Gold Mine [Ref 3]
 

Figure 2. Weavers Needle, the landmark for the location of the Lost Dutchman Gold Mine [Ref 2].
 

Two roots for the of name of the lost El Naranjal Mine have been attributed to its location near a grove of trees with oranges (naranjas) or to the orange color of the gold in its ore [Ref 3]. It is supposedly located at the bottom of canyon (Barranca) beside a river and near an abandoned hacienda.

Among many of the discussions about this mine, Treasurenet suggests that proof of the its existence lies in an old road sign naming the road to the mine in Sinaloa and in records found in Guadalajara, which were found by a British consul, describing production in the millions in the 17thCentury [Ref 4]. In another posting,

TreasureNet, [Ref 5], suggests its location fits that of an 1800’s lost, and very rich gold mine, in the region of the lost Tayopa silver mine [Ref 6], and in another post [Ref 7] that its location lies in the state of Durango.

Ref 1. http://www.thegeozone.com/treasure/arizona/index.jsp

Ref 2.  http://treasure-hunting-information.com/?page_id=2641

Ref 3. https://en.wikipedia.org/wiki/Naranjal_mine

Ref 4. http://www.treasurenet.com/forums/tayopa/36414-el-naranjal.html

Ref 5. http://www.treasurenet.com/forums/tayopa/273860-can-el-naranjal-possibly-found-tayopa-complex.html

Ref 6. http://www.treasurenet.com/forums/tayopa/36414-el-naranjal.html

Ref 7. http://www.treasurenet.com/forums/treasure-legends/487274-update-mine-el-naranjal.html