Chemical composition -- A silicate of magnesium and iron.
Color -- Green to yellow-green, rarely brown.
Optics -- R.I. 1.65-1.69. Sometimes higher. Birefringence is high (0.036), and careful orientation is needed to produce the finest faceted stones.
Durability -- Hardness 6.5-7.
Crystal structure -- Orthorhombic.
Specific Gravity -- 3.2-3.5.
Sources -- Major suppliers are the Navajo Reservation in New Mexico and Arizona, Burma, Egypt (Zebirget), Burma, Mexico, Norway, and Kenya. Small stones are abundant and inexpensive, but large, fine stones are scarce and moderately expensive.