Both precious red and black corals are bottom-dwelling sessile marine branched animals that have been harvested at depths between 60 and 20,000 feet [Ref 1, Ref 2]. They have provided lapidarists and other artisans with materials they use to create extraordinarily beautiful jewelry and objects of art, as witnessed by Figures 1 and 2. In this Blog, I’ll describe the physical properties and the sources of the colors of the corals that underlie their use in jewelry making and in carving fine objects of art.
As shown in Figure 1, the Chinese Qing Dynasty red coral sculpture, featuring four carved figures of Buddha, capitalizes on the flowing branched nature of the coral to produce a beautiful and graceful carving. The flowing shapes of the branches
of black coral are an interesting contrast to the disciplined shapes of the gold fittings set with gems, as seen in Figure 2.
PROPERTIES OF PRECIOUS RED AND BLACK CORALS
Gemstones of red and black coral comprise the hard tissue core or skeleton which supports the soft tissues of the organisms as shown in Figures 3-5.
PROPERTIES OF PRECIOUS RED AND BLACK CORALS
Gemstones of red and black coral comprise the hard tissue core or skeleton which supports the soft tissues of the organisms as shown in Figures 3-5. The physical and chemical properties of each coral underlie its usage in jewelry and art objects.
Physical and Chemical Properties of Precious Red Coral
The hard tissue of the skeleton of red coral is comprised of aggregated nanometer-sized crystals of magnesium-rich calcite {(Ca.Mn)CO3} which are arranged in rings separated by an annular layer of organic matrix containing glycoproteins and glycosaminoglycans[Ref 8]. The annular arrangement of the mineral and organic matrix is shown in the cross-section of a stem in Figure. The organic matrix is highlighted by a purple stain and the layer of magnesium-rich calcite is unstained [Ref 9]. The axial region rich in magnesian calcite is indicate by the letter
A and the spaces occupied by the longitudinal canals, the regions of soft tissue which transport nutrients are labeled by the letter B.
Pigmentation in Precious Red Coral
The red pigment in precious red coral has been determined to be canthaxanthin, a member of the family of retinoids which are ubiquitous in red-colored organisms [Ref 11]. Its location in the outer soft tissue and organic matrix is demonstrated by its red color in Figures 7 and 8. The range of hues of the red color of the coral as a gemstone is shown in Reference 12.
Mechanical Properties of Precious Red Coral [Ref 14]
The values of Mohs Hardness in the range 3-4 for red coral are comparable to that of 3 for black coral [Ref 14]. With its axial core of magnesian calcite, red coral is brittle with an irregular or splintery fracture, and cannot be bent to assume complex shapes as can be done with black coral. Natural shapes and those obtained by carving are similar to those in black coral jewelry and art objects as can be seen in comparing the jewelry and art objects in Figures 9-17 to those in Figures 18-26.
RED CORAL JEWELRY AND ART OBJECTS
[Ref 17].
Physical and Chemical Properties of Black Coral
Some black corals are branched, while others have long and straight stems or spirally twisted stems, as shown in Figures 18 and 19. The stem or branch of the black coral skeleton is not composed of a core of magnesian-rich calcite as in red coral, but consists of laminated composites comprised primarily of protein and chitin fibrils [Ref 24]. The growth of the diameter of a stem or branch proceeds by accretion of layers, as shown by the polished cross section of a stem in Figure 18 [Ref 25]. The black color seen throughout the stem is due to the melanin [Ref 25], a black pigment widely distributed in both the animal and plant kingdoms [Ref 26].
showing layers of chitin after removal of proteins. The fine structure of
concentric rings is evident. The diameter of the stem is ~ 7 mm {Ref 29].
Mechanical Properties of Black Coral
To compensate for the lack of stiffness, due to the absence of a skeletal core of rigid magnesium, as in red coral layers, the chitin fibrils [Ref 30] comprising the strong part of the skeleton, is spirally wound in the concentric layers [Ref 30] comprising the stem and branches as shown in Figures 8-10. Such a winding scheme works to prevent kinking of the stem due to bending and twisting of the stem due to torsional forces. This scheme compensates for the lack of a stiff mineral axis as is present in red coral. Values of the mechanical measures of the strength of the skeleton of black coral are summarized in Table I taken from Ref 30.
The fibrils are wound in spiral fashion about the axis of the stem. The scale bar
is 10 microns (0.0010 cm) [Ref 30].
TABLE I MECHANICAl PROPERTIES OF SKLETONS OF TWO SPECIES OF BLACK CORAL (Antipathians)[Ref30]
| PROPERTIES | ANTIPATHES FIORDENSIS | ANTIPATHES SALIX |
| Mohs Hardness | 3 | 3 |
| Micro-indentation Hardnesshardness along the long axis (pounds/mm2) | 26,000 lb/in2 | 31,663 lb/in2 |
| Micro-indentation hardness along a diameter (pounds/mm2) | 28,968 lb/in2 | 32,516 lb/in2 |
| Extensibility (%) | 7.37 | 3.84 |
The Mohs hardness of 3, of black coral, is comparable to the range of values 3-4 of red coral, so that expectations of resistance of both to wear, particularly scratching are similar. Values of micro-indentation hardness in the range of 26,000 to 32,516 lb/in2 are approximately one-half the value of 58,064lb/in2 estimated for the rhombohedral surface of calcite estimated from Figure 1. These values of Mohs and micro-indentation hardness suggest care in wearing black coral jewelry. Jewelry such as pendants, earrings, and necklaces of this coral will have longer scratch-free life expectancy.
The large room-temperature values of the extensibility of the two black coral species of 3.845% and 7.37%, especially augmented by the thermoelasticity, [Ref 31], of the coral at higher temperatures underlie the shaping of jewelry with intricate shapes. The softness of black coral underlies the use of carved shapes in jewelry. Working the coral at an elevated temperature would further enable attaining intricate shapes with finer carved relief.
REFERENCES
Ref 1. https://coral.org/coral-reefs-101/coral-reef-ecology/how-coral-reefs-grow/
Ref 2. https://ocean.si.edu/ecosystems/coral-reefs/deep-sea-corals
Ref 3. http://www.alaintruong.com/archives/2013/11/09/28391538.html
Ref 4. https://www.pinterest.com/pin/57702438948445952/
Ref 5. https://www.gia.edu/doc/Spring-2007-Gems-Gemology-Pink-Red-Coral-Guide-Determining-Origin-Color.pdf
Ref 6. https://www.gia.edu/doc/Spring-2007-Gems-Gemology-Pink-Red-Coral-Guide-Determining-Origin-Color.pdf
Ref 7.https://www.advancedaquarist.com/2014/11/corals
Ref 9. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3142144/
Ref 12. https://www.gemdat.org/gem-42717.html
Ref 13.
Ref 14. https://en.wikipedia.org/wiki/Precious_coral
Ref 15. https://www.shellhorizons.com/details.asp?ProductID=CR1-2&Page=1
Ref 16. https://www.enjoythecoast.it/en/the-coral-jewelry
Ref 17. https://jjpjewelry.com/products/good-luck-coral-horn
Ref 18. https://www.pinterest.com/pin/480829697690763070/
Ref 19. https://www.pinterest.com/pin/480829697706273935/
Ref 20. http://ahwilkens.com/portfolio/chinese-finely-carved-red-coral-figure-mulan/
Ref 22. https://www.amazon.com/Arthurs-Jewelry-Genuine-natural-enhancer/dp/B01H4U0DSM
Ref 23. https://www.qvc.com/American-West-Red-Coral-Carved-Rose-Sterling-Ring.product.J271747.html
Ref 25.
Ref 26. https://www.ncbi.nlm.nih.gov/pubmed/9451820
Ref 27. https://www.flickr.com/photos/searchoflife/12464176455
Ref 28. https://www.advancedaquarist.com/2014/11/corals
Ref 30.https://www.jstor.org/stable/1542113?read-now=1&seq=14#metadata_info_tab_contents
Ref 33. https://www.collectorsweekly.com/stories/127806-black-coral-cuff-bracelet
Ref 34. https://www.mauidivers.com/collections/black-coral-jewelry
Ref 36. https://www.mauidivers.com/collections/black-coral-jewelry
