Cassiterite
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Cassiterite is a tin oxide mineral, SnO₂. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem. Cassiterite was the chief tin ore throughout ancient history and remains the... Wikipedia
Birefringence: δ = 0.103
Cleavage: imperfect,, indistinct; partings on, or
Color: Black, brownish black, reddish brown, brown, red, yellow, gray, white; rarely colorless
Crystal class: Ditetragonal dipyramidal (4/mmm); H-M symbol: (4/m 2/m 2/m)
Crystal habit: Pyramidic, prismatic, radially fibrous botryoidal crusts and concretionary masses; coarse to fine granular, massive
Crystal system: Tetragonal
Fracture: Subconchoidal to uneven