Coconino Lapidary Club




Corundum – From Sapphires to Rubies


Gemstones of the mineral corundum [Ref 1] offer a rainbow of colors for the lapidarist and jewelry maker as displayed in Figure 1. 

Traditionally, of these, the ruby and blue sapphire, along with diamond and emerald, are considered to be the four-membered family of precious gems.  Corundum gemstones, other than the ruby and blue sapphire, are also considered sapphires, having colors ranging from green to pink.

In this blog, I’ll describe the crystallography of corundum, and the physical and optical properties of corundum, including the sources of the colors in its gemstones. I will also present a gallery of ruby and sapphire mineral specimens.

Figure 1. A rainbow of the gemstones from corundum: ruby, blue sapphire, and green to pink sapphires.


Crystal System of Corundum

Corundum crystallizes in the Trigonal System, which has three axes in a plane and are arranged at 120 degrees to each other, with an axis perpendicular to the plane, as shown in Figure 2. Of the typical forms of crystals shown in the figure, corundum frequently crystallizes as a hexagonal prism, terminated by the basal pinacoid; as a bipyramid, the hexagonal prism is terminated by a bipyramid; the rhombehedron and the hexagonal prism are terminated by the rhombehedron, and the schalenohedron. Examples of corundum crystals taking these forms are shown in Figures 5-11. Figure 7 shows a diagram of a crystal exhibiting all of these forms except the rhombohedron and schalenohedron. The latter form is shown by the sapphire crystal in Figure 10.

Figure 2. The four axes of the trigonal crystal system
Figure 3. Properties of the Trigonal Crystal System
Figure 4. Crystal of corundum with typical forms; only the schalenohedron is not shown.

Twinning in Corundum [Ref 1].

Multiple twinning on the rhombohedral plane with laminar structure with striations on both the basal pinacoid perpendicular to the c-axis and the hexagonal prism or on bipyramid faces, as shown by the terminated bipyrimidal sapphire crystal, shown in Figure 10 [Ref 12]. Corundum is also twinned on the hexagonal prism faces of tabular crystals exhibiting an arrowhead shape, as shown by the sapphire specimen in Figure 11[Ref 13]. The view is at the base of the arrowhead shape and pointing towards the tip. Less frequent twinning in corundum occurs on the basal pinacoid, perpendicular to the long axis of the crystal, as showing repetitive twinning along its length in Figure 12. 

The view is at the base of the arrowhead shape and pointing towards the tip. Less frequent twinning in corundum occurs on the basal pinacoid, perpendicular to the long axis of the crystal, as showing repetitive twinning along its length in Figure 12. 

The view is at the base of the arrowhead shape and pointing towards the tip. Less frequent twinning in corundum occurs on the basal pinacoid, perpendicular to the long axis of the crystal, as showing repetitive twinning along its length in Figure 12. 


The high values of hardness and ultimate strength and its resistance to cleavage, underlie the toughness of corundum gemstones and their wide usage in rings and bracelets, both susceptible to impact while worn. Values of the strength factors of corundum are summarized in TABLE I.


Mohs Hardness SCALE9 (Member of scale)11
Ultimate Compressive Strength435,000 psi13
Ultimate Tensile Strength43,500 psi13
Flexural Strength58, 00013
CleavageNone Observed11


The Refractive Index values of corundum lie in the ranges 1.759-1.772 depending on direction of light polarization. These values are considerably below the value of 2.418 for diamond [Ref 3], and underlies the beauty of corundum gemstones being in their vivid colors and not in brilliance or fire.

The light reflected from the surface, without penetration into gemstones is colorless, as often seen in photographs of gemstones, as in Figure 14.

Light scattering from oriented needle-like crystals of rutile, or to colloidal or other material in oriented tubules is observed in the star sapphire and star ruby as described in another blog on star rubies and sapphires [Ref 4].

Figure 14. Reflections from the surface of the ruby gemstone are colorless, while those reflected from the back of the stone are colored.


Corundum is aluminum oxide, with the formula Al2O3. Each trivalent aluminum Al3+ ion is surrounded by six oxygen ions, located at the tips of an octahedron in the crystal lattice of corundum, shown in Figure 15. Defects in the forms of ions of metal impurities substituting for the aluminum ion, are responsible for the colors of corundum [Ref 5 ]. The impurity metal ions and the associated colors are summarized in Table I, shown in Figure 4. The divalent and trivalent ions substitute for the aluminum ion in the lattice of the corundum lattice.

Figure 15. Crystal lattice of corundum.
Figure 4. Sources of colors in corundum gemstones. See legend for terms below.

Legend for Figure 4
Cr3+ = Trivalent chromium ion 
Fe3+ = Trivalent iron ion
Fe2+ = Divalent iron ion
Ti4+ = Tetravalent titanium ion
O1-V = neighboring monovalent oxygen ion O1- and lattice vacancy V in lattice
            taking the place of an Al3+ ion.
Al3+ = Trivalent aluminum ion


Consideration of the various colors in natural sapphires, having different combinations and concentrations of the ions and ion pairs, before and after their heat treatment, serves to demonstrate their effects on color in corundum gemstones. The results of heat treatments are shown in Figures 16-18.

Some sapphires are heat treated to improve the attractiveness of their colors. Studies were carried out to identify changes in concentrations of ions that led to improvements in the aesthetics of the gem stone. The studies showed two major effects in the brown-toned sapphires and in the optical absorption spectrum of sample rO 4/5, red orange. The red trace of the absorption spectrum shows increased absorption due to the chromium ion, a decreased absorption due to trivalent iron ion pairs contributed from paired divalent and trivalent iron ions and single trivalent iron ions. The heat treatment resulted in an increased number of paired divalent iron ions and tetravalent titanium ions. The lessened absorption by iron ions resulted in smaller contributions to the color of the gemstone in the yellow to orange spectral range. Increased trivalent chromium ion concentration resulted in increased absorption of the blue and yellow spectral range and increased transmission in the red spectral range. Increased absorption in the yellow-orange range, due to increased absorption by paired divalent iron and tetravalent titanium ions resulted in increased transmission in the blue spectral range. The lessened transmission in the yellow-orange and increase transmission in the red and blue color ranges resulted in the cherry-pink color of the gemstone.

Figure 16. Ranges of colors in sapphires obtained with heat treatment under reducing conditions, within two temperature ranges.
Figure 17. Samples of sapphires before and after heat treatments at 1100-1700 
Degrees C. Samples before treatment are shown in the top row and after treatment shown in the following rows.
Figure 18. Changes in light absorption and transmission in a sapphire with a red-orange color before treatment and a cherry-pink color achieved after treatment.


Many specimens on display are from alluvial deposits where erosion of the edges and faces arose from wear against surrounding gravel and sand.

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