Amarantite

Amarantite is an amaranth-red to brownish mineral with the general formula of Fe₂O(SO₄)₂·7(H₂O) or Fe(SO₄)(OH)·3(H₂O).

The name comes from the Greek word αμάραντος which means amaranth, an imaginary undying red flower, in allusion to its color.

Amarantite is triclinic, which means crystallographically, it has only one symmetry fold. It must be rotated 360 degrees to be exactly the same. Due to it being triclinic it falls into the biaxial optical class, the axis degrees do not equal 90 degrees and the sides of each axis are not the same length. Amarantite is anisotropic, which means, the velocity of light varies with crystallographic direction, and there is more than one refractive index.