Its name comes from the greek for cross. The twin is a classic penetration twin where it appears as if two crystals grew into and out of each other. The 60 degree type is more common but the 90 degree type is the most sought after. It was associated with Christianity as a symbol for the cross and as a good luck charm. Some scarcer specimens show both twin forms and can look like a blunt six rayed star.
Color is reddish-brown, brown, and black.
Luster is vitreous to resinous to dull. Transparency crystals are translucent to opaque. Crystals sometimes grown onto kyanite crystals. Cleavage poor, in one direction. Best Field Indicators are color, associations, twinning and crystal habit.