Corundum

Hardness: 9 (Mohs scale), Color: Varies widely, including red (ruby), blue (sapphire), and other colors (fancy sapphire). The specimen appears to be a light purplish-gray; however, a red hue is visible on the top edge, suggesting it might be a low-grade ruby or corundum with inclusions. Luster: Vitreous to adamantine. Crystal structure: Hexagonal. Cleavage: None (parting along basal planes or rhombohedral planes can occur due to twinning or pressure). Specific Gravity: 3.95-4.10.

Corundum forms in igneous, metamorphic, and sometimes sedimentary environments. It commonly crystallizes in aluminum-rich, silica-poor igneous rocks (like syenites and nepheline syenites) and pegmatites, as well as in metamorphic rocks (schists, gneisses, marbles) formed from alumina-rich sediments. It can also be found in placer deposits, eroded from its original host rock due to its high hardness and density. Geological age varies depending on the specific deposit, ranging from Precambrian to Cenozoic.

Corundum is highly valued as a gemstone (ruby and sapphire) for jewelry. Its extreme hardness makes it useful as an abrasive in sandpaper, grinding wheels, and other cutting tools. Synthetic corundum is also manufactured for these purposes and for watch bearings, laser components, and other industrial applications.

Corundum is the second hardest natural mineral after diamond. Red corundum is known as ruby, and all other colors of corundum are called sapphire. Trace elements give corundum its color; for example, chromium creates red in ruby, and iron and titanium create blue in sapphire. Star rubies and sapphires exhibit asterism due to oriented rutile inclusions.

In the field, corundum often occurs as hexagonal barrel-shaped crystals. Its high hardness can be tested by scratching other, softer minerals. The specimen shows a crystalline structure with a reddish tint on one side, hinting at a ruby or corundum specimen with various inclusions, likely silicates judging by the gray-green matrix. It is commonly found in Myanmar (Burma), Sri Lanka, Thailand, Australia, India, Madagascar, Tanzania, and the United States (Montana, North Carolina). Collectors often look for good crystal habit, color saturation, and clarity. The visible greenish and yellowish speckles within the matrix suggest this is corundum in its host rock, likely a marble or schist.