Chemical composition -- Beryllium borate with hydroxyl and fluorine.
Color -- Colorless to yellowish or grayish white.
Optics -- R.I 1.55-1.63. Birefringence very high (0.072).
Durability -- Hardness 7.5. Brittle, with one direction of perfect cleavage. Durable enough for use in jewelry (but usually relegated to collections due to lack of color and brilliance).
Crystal structure -- Orthorhombic.
Specific Gravity -- 2.35-2.37 (very low specific gravity for materials with such high birefringence).
Sources -- Madagascar is probably the best source, but hambergite remains rare, especially in finished stones over 5 carats. The largest known stone is about 40 carats.