Red Spinel Crystal on Dolomite

Red Spinel Crystal on Dolomite from Vietnam. 

Measures approximately 1.7 x 1.5 x 0.7 inch

Spinel usually occurs in isometric crystals, octahedrons, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Spinel is found as a metamorphic mineral, and also as a primary mineral in basic rocks,because in such magmas the absence of alkalis prevents the formation of feldspars and any aluminium oxide present will form corundum or combine with magnesia to form spinel. The name "spinel" is derived from the Greek word for spark, in reference to the fiery red color of spinels often used for gems. The transparent red spinels are also called spinel-rubies or balas-rubies and were often confused with actual rubies in ancient times.