ClipArt ETC
"The mineral corundum which includes the precious stones ruby, sapphire, and topaz, contains alumina, or aluminum oxide, Al2O3. It is found in nature crystallized in rhombohedral forms, having a specific gravity of 3.9, and is scarcely inferior in hardness to diamond." —Hallock 1905

Aluminum

"The mineral corundum which includes the precious stones ruby, sapphire, and topaz, contains alumina,…

Corundum.

Corundum

Corundum.

Corundum.

Corundum

Corundum.

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." — Ford, 1912

Corundum

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." —…

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." — Ford, 1912

Corundum

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." —…