Hardness, high specific gravity, six-sided crystals sometimes tapering to a pyramid, parting, luster, conchoidal fracture Corundum does display parting perpendicular to the c-axis.ģ.9 to 4.1 (very high for a nonmetallic mineral) Chromium produces red (ruby) and combinations of iron and titanium produce blue (sapphire). Colorless when pure, but trace amounts of various metals produce almost any color. A summary of the physical properties of corundum is given in the table below. Its hardness, high specific gravity, six-sided crystals and parting are very good diagnostic properties to use in its identification. It serves as the index mineral for a hardness of nine on the Mohs Hardness Scale. It is the third-hardest mineral, after diamond and moissanite. Properties of CorundumĬorundum is an exceptionally hard and tough material. This specimen is about twelve centimeters across and has a round blue sapphire crystal on the left side. They are used to make industrial bearings, scratch-resistant windows for electronic instruments, wafers for circuit boards, and many other products.Ĭorundum gneiss with sapphire: A specimen of corundum gneiss from Gallatin Valley, Montana. Natural and synthetic corundum are used in a wide variety of industrial applications because of their toughness, hardness, and chemical stability. The extreme hardness makes corundum an excellent abrasive, and when that hardness is found in beautiful crystals, you have the perfect material for cutting gemstones. The mineral is widely known for its extreme hardness and for the fact that it is sometimes found as beautiful transparent crystals in many different colors. It is an aluminum oxide with a chemical composition of Al 2O 3 and a member of the trigonal crystal system.

What is Corundum?Ĭorundum is a rock-forming mineral that is found in igneous, metamorphic, and sedimentary rocks. These specimens are red in color and might be called 'ruby corundum.' Image copyright iStockphoto / Lissart. Corundum: Two corundum crystal segments from India showing the mineral's six-sided crystals with basal parting.

Corundum blue