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Rough corundum.
All gems are cut and polished by progressive abrasion using finer and finer grits
of harder substances. Diamond, the hardest naturally occurring substance, has
a Mohs hardness of 10 and is used as an abrasive to cut and polish a wide variety
of materials, including diamond itself. Silicon carbide, a manmade compound of
silicon and carbon with a Mohs hardness of 9.5, is also widely used for cutting
softer gemstones. Other compounds, such as cerium oxide, tin oxide, chromium
oxide, and aluminum oxide, are frequently used in polishing gemstones. |
