The Calcium Sulfate Mineral, Gypsum, is a very common mineral, and occurs in many parts of the world. Gypsum crystals are clear to colorless (such as the specemin in the photo. Most gypsum is white to a light gray in color, and typically has 20% water in its structure. It has a SG of 2.3 and a hardness of 2. It is easily scratched by a fingernail. Gypsum is a very important commercial mineral, and is present in almost all buildings as wallboard or plaster or both. It usually originated from precpitation from saline water, and the minerals that precipitated out with the gypsum will determine the color of the mineral. Another use of gypsum is for insulation and a fire retardant. It's crystal structure is monoclynic and it has tabular or rectangular crystals.