Chemical composition -- A complex family of iron and magnesium silicates.
Color -- Usually green, brown, or yellow.
Optics -- R.I. 1.65-1.77 (usually 1.65-1.68).
Durability -- Hardness 5-6.
Crystal structure -- Orthorhombic.
Specific Gravity -- 3.2-4.0.
Sources -- Blackish green four-ray star enstatite cabochons from India and Sri Lanka are plentiful and inexpensive. They are often used as cheap (and much softer) substitutes for black star sapphires.