spinel

also spi·nelle (spĭ-nĕlˈ)

1. 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.
2. Any of a group of minerals that are oxides of magnesium, iron, zinc, manganese, or aluminum.