THE MINERAL CORUNDUM

  • Chemical Formula: Al2O3, Aluminum Oxide
  • Class: Oxides and Hydroxides
  • Group: Hematite
  • Uses: As a gemstone, mineral specimens and as an abrasive.
  • Specimens
      Also see variety specimens:
    • Ruby Specimens
    • Sapphire Specimens
When someone lists the most famous gemstones such as diamond, topaz, aquamarine, emerald and garnet, corundum does not usually get mentioned. However, its two varieties are sure to be on any list of gemstones. The red variety of corundum is known as ruby and all the other colors of corundum are known as sapphire. The rare colorless or white crystals are called corundum in the mineral world, but artificial colorless plates of corundum are called sapphire and used as watch faces for high-end timepieces. That's because a sapphire crystal sounds much more valuable than a corundum crystal.

Corundum is the third hardest natural mineral known to science. The hardest mineral, diamond is still four times harder than corundum. The second hardest is Moissanite (Silicon Carbide) at 9.25 barely beats corundum's hardness of 9.0. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.

PHYSICAL CHARACTERISTICS:

  • Color is highly variable. The color can be white or colorless, blue, red, yellow, green, brown, purple, and pink; there are also instances of color zonation.
  • Luster is vitreous to adamantine.
  • Transparency: Crystals are transparent to translucent.
  • Crystal System is trigonal; bar 3 2/m
  • Crystal Habits include sapphire's typical six-sided barrel shape that may taper into a pyramid, and ruby's hexagonal prisms and blades.
  • Cleavage is absent, although there is parting which occurs in three directions.
  • Fracture is conchoidal.
  • Hardness is 9
  • Specific Gravity is approximately 4.0+ (above average for translucent minerals)
  • Streak is white.
  • Associated Minerals include calcite, zoisite, feldspars, micas and garnets.
  • Other Characteristics: Refractive index is around 1.77; pleochroic (meaning color intensity is variable from different viewing directions); striations on parting surfaces.
  • Notable Occurrences include Burma; Sri Lanka; North Carolina and Montana, USA; many African localities; several localities in India, and Middle Eastern and Southeast Asian countries.
  • Best Field Indicators are extreme hardness, density and crystal habit.
CORUNDUM specimens:
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CORUNDUM specimen cor-1
$ 90.00
Dims: 4.3 x 2.2 x 2.2" (10.9 x 5.6 x 5.6 cm)
Wt: 13.9 oz. (393.0 g)
Tanzania
Maybe I should have classified this specimen as a "ruby" specimen, but I did not think that the color and quality were sufficient. This hand specimen consists of several pink Corundum crystals that are partly-embedded in a matrix of blended quartz, feldspars, and biotite mica. They are small, not exceeding 0.7" (1.8 cm) in length or 0.3" (0.8 cm) in diameter, and a few are noticeably damaged or broken. They do have excellent hexagonal prismatic form, however, with well-defined edges and reasonably clean faces that possess a waxy-to-pearly luster. All are very cloudy and translucent, and likely contain many basally-oriented internal fractures. The host rock is mostly very granular, but it appears to solidify and become more purley feldspar-bearing immediately surrounding the Corundum crystals.
no photo
cor-1 ($ 90.00)
Tanzania
CORUNDUM specimen cor-2
$ 30.00
Dims: 0.7 x 0.6 x 0.2" (1.8 x 1.5 x 0.5 cm)
Wt: 12.2 g
Afghanistan
A small, cut and partly-polished section of a Corundum crystal makes up this piece. It has the standard hexagonal form and a pale violet-red coloration and is translucent. Where it is not cut, the crystal is coated by a thin crust of green zoisite that is also translucent. It is hot-glued onto the foam base of a plastic specimen box. Such a piece would make an excellent stone for a pendant.
no photo
cor-2 ($ 30.00)
Afghanistan
CORUNDUM specimen cor-3
$ 57.00
Dims: 1.2 x 0.8 x 0.7" (3.0 x 2.0 x 1.8 cm)
Wt: 11 g
Ganesh, Hunza River, Hunza, Northern Areas, Pakistan
Two distinct Corundum crystals rest in the pegmatite matrix of this thumbnail specimen. The larger of the crystals has dimensions of 0.4 x 0.3 x 0.3" (1.1 x 0.8 x 0.7 cm) and is lightly damaged in one area, whereas the smaller one is less than half that size and in excellent condition. The smaller crystal is quite warped, as it is intergrown with the larger- the larger is lightly warped and incomplete, but still has a rather well-defined hexagonal form. Both have a red-violet coloration and a greasy luster, and both are translucent and cloudy, containing many internal fractures or cleavage planes. A single, small muscovite book is also intergrown with the larger Corundum crystal.
no photo
cor-3 ($ 57.00)
Ganesh, Hunza River, Hunza, Northern Areas, Pakistan
CORUNDUM specimen cor-4
$ 27.00
Dims: 1.1 x 0.9 x 0.5" (2.7 x 2.2 x 1.2 cm)
Wt: 0.4 oz. (10 g) w/ foam base
Afghanistan
A partial "nodule" consisting of a Corundum core and a zoisite shell comprises this thumbnail piece. The only visible portion of the Corundum has been polished, so damage assessment is impossible. Its crystal form cannot be defined, either. The Corundum has a dull violet color and is translucent, containing many fibrous inclusions. The zoisite has a dull green color and is likewise translucent. The piece is hot-glued inside a plastic thumbnail box. Further cutting could produce a suitable stone for a pendant.
no photo
cor-4 ($ 27.00)
Afghanistan
CORUNDUM specimen cor-6
$ 60.00
Dims: 3.1 x 0.5 x 0.5" (7.9 x 1.3 x 1.3 cm)
Wt: 0.8 oz. (23 g)
near Mount San Jacinto, Riverside County, California, U.S.A.
A single, large Corundum crystal comprises this specimen. Though broken and repaired, it has an excellent appearance. Its trigonal form is nearly perfect and its habit six-sided and pyramidal, tapering very steeply to a point. Though a small surface at its tip is uneven, it appears to be a natural flaw or very old, healed damage. There is no host rock present, and the crystal is affixed to a flat acrylic base with a removable putty.
no photo
cor-6 ($ 60.00)
near Mount San Jacinto, Riverside County, California, U.S.A.
CORUNDUM specimen cor-5
$ 42.00
Dims: 1.2 x 0.8 x 0.6" (3.0 x 2.0 x 1.6 cm)
Wt: 0.4 oz. (119 g) w/ foam base
Hit-Ostrov, Karelia, Kola Peninsula, Russia
A single Corundum crystal extends from the syenite base of this thumbnail piece. This crystal has dimensions of 0.6" length x 0.3" width (1.4 x 0.6 cm) and is in excellent condition. Its hexagonal form is likewise excellent, though its disjointed termination suggests breakage and healing during formation. Its dull, red-violet color and greasy luster are common for such specimens from this locality, and its is dimly to moderately translucent. Like many syenites, the base appears to contain either many other, tiny Corundums or perhaps tiny garnets. The piece is hot-glued into a plastic thumbnail box.
no photo
cor-5 ($ 42.00)
Hit-Ostrov, Karelia, Kola Peninsula, Russia
 

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