Gem Minerals
Corundum
What it is
Pure aluminum oxide, and the second-hardest natural mineral after diamond (Mohs 9). In its pure form corundum is colorless, but trace chromium makes it red (ruby) and iron-titanium makes it blue (sapphire). All other colors are also called sapphires.
How it forms
Crystallizes in aluminum-rich, silica-poor igneous and metamorphic rocks. The key requirement is an environment rich in aluminum but low in silicon — if silica is present, it combines with aluminum to form feldspar instead.
Al₂O₃ (Trigonal)
