Corundum

Al₂O₃

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Second in hardness only to diamond, corundum is an aluminum oxide (Al2O3) with a Mohs hardness of 9. Its gem varieties are ruby, colored red by chromium, and sapphire, which covers all other colors. The mineral crystallizes in the trigonal system as tabular, prismatic, or bipyramidal crystals with an adamantine luster, a colorless streak, and no cleavage. Specific gravity ranges from 3.98 to 4.10. Corundum's extreme hardness makes it a widely used abrasive, and synthetic corundum serves in laser technology, while the finest natural crystals are cut as ruby and sapphire. Colors other than red span the full range of sapphire, from blue and yellow to pink and colorless.

Corundum is the second hardest natural mineral after diamond. Its gem varieties are ruby (red, colored by chromium) and sapphire (all other colors). Synthetic corundum is widely used as an abrasive and in laser technology.

晶系	Trigonal
矿物分类	04 Oxides and Hydroxides
莫氏硬度	9.0
比重	3.98-4.10
颜色	Red (ruby), blue (sapphire), various colors
光泽	Adamantine
条痕色	Colorless
解理	None
断口	Conchoidal to uneven
透明度	Transparent
晶体习性	Tabular, prismatic, bipyramidal