Description
From Wikipedia: As the most common source of mercury in nature, Cinnabar generally occurs as a vein-filling mineral associated with recent volcanic activity and alkaline hot springs. The mineral resembles quartz in symmetry and in its exhibiting birefringence; cinnabar has a mean refractive index of ~3.2, a hardness between 2 and 2.5, and a specific gravity of ~8.1. The color and properties derive from a structure that is a rhombohedral crystalline lattice belonging to the hexagonal crystal system, crystals that sometimes exhibit twinning.