Bismutite
>
Bismutite or bismuthite is a bismuth carbonate mineral with formula Bi₂O₂. Bismutite occurs as an oxidation product of other bismuth minerals such as bismuthinite and native bismuth in hydrothermal veins and pegmatites. It crystallizes in the... Wikipedia
Cleavage: Distinct/Good on(microscopically observable)
Color: Yellow to brown, greenish, green-grey, grey or black
Optical properties: Biaxial (−) (appears uniaxial due to twinning)
Refractive index: a=2.12–2.15, b=2.12–2.15, g=2.28