A mineral occuring in octahedrons of great hardness and various colors, as red, green, blue, brown, and black, the red variety being the gem spinel ruby. It consist essentially of alumina and magnesia, but commonly contains iron and sometimes also chromium.
- A hard, variously colored mineral with composition MgAl2O4, having usually octahedral crystals and occurring in igneous and carbonate rocks. The red variety is valued as a gem and is sometimes confused with ruby.
- Any of a group of minerals that are oxides of magnesium, iron, zinc, manganese, or aluminum.