Spinel
ID: 18
Spinel
Chemical Formula
MgAl₂O₄
Hardness (Mohs)
7.5 - 8
Crystal System
Isometric
Streak
White
Luster
Vitreous (glassy)
Diaphaneity
Transparent to opaque
Color
Red (most famous), blue, pink, purple, black, green, brown, colorless.
Crystal Habit / Mode of Occurrence
Isometric - Hexoctahedral; commonly octahedral, often twinned; also massive, granular.
Fun Facts
Many famous "rubies" in historical crown jewels, such as the "Black Prince's Ruby" in the British Imperial State Crown, were later identified as large red spinels. Common Uses: Gemstone (especially red and blue varieties), refractory material (high heat resistance), historical use as a pigment (spinel black).