Proustite

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Chemical composition -- A compound of silver, arsenic, and silicon.

Color -- Intense deep red.

Optics -- R.I 2.79-3.09. Birefringence very high (0.296). Adamantine to submetallic luster.

Durability -- Hardness 2-2.5. Brittle. Much too soft for jewelry use.

Crystal structure -- Hexagonal.

Specific Gravity -- 5.57-5.64 (very dense).

Sources -- Proustite is found in silver mines, notably from Chile, Mexico, and Ontario, but is quite rare, especially in finished stones. While much too fragile for use in jewelry, it is a prized collector's item due to its intense color and brilliance. While large crystals are known, cut stones are always rather small (several carats) due to the difficulty of cutting and to the desirability of large crystal specimens.

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