Gypsum

Two samples of colourless Gypsum with one sample having slight yellow tints. The colourless/yellow sample has a subhedral arrow shape form with a vitreous lustre. The other sample is subhedral rhombic form with a vitreous lustre and pearly lustre on the cleavage planes. Gypsum (CaSO4 x 2H2O) can be white, colourless, grey, yellow, red or brown in colour. Lustre is vitreous, but pearly on the cleavage and streak is white. Hardness is 1 1/2 to 2 and cleavage is perfect in one direction and good in two directions. Gypsum is usually found as untwinned single crystals that show a rhombic form but can also be twinned arrowhead shape, granular, massive, fibrous (satin spar) or earthy.