Spinel - MgAl₂O₄

Habit: Crystals are usually octahedrons, but can be granular or massive too, and range in color from brown, black; red (ruby-spinel), orange, yellow, green, blue, indigo, violet or colorless. Transparent to nearly opaque, Vitreous, splendent to dull Luster.

Environment: Formed at high-temperature as an accessory in igneous rocks, in regionally metamorphosed aluminum-rich schists and in regionally and contact metamorphosed limestones. Also is a detrital mineral

Etymology: From the Latin spinella, for little thorn, due to the spine-shaped octahedral crystals.