Corundum
Corundum is the second hardest natural mineral known to science (1/4 the hardness of diamond). Gem varieties are sapphire and ruby.Corundum may occur on a large scale in some pegmatites. It is also found in silica-poor hornfelses (a contact metamorphic rock).
corundum (sapphire) |
Chemical composition - Al
2O
3
Other specimens - Click the thumbnails to enlarge |