Gypsum, on its smallest scale, looks like this.
Gypsum, a hydrated calcium sulfate (chemical formula Ca[SO4]•2H2O), is a commercially versatile mineral. It is found in a range of colors and shapes, in many countries, both above and below ground.
Shape
Gypsum forms as granular masses, fibrous masses, tabular shapes, rosettes and prisms. Gypsum is also known to twin, to form mirror image crystals on the same level. It is classified under the monoclinic crystal system.
Colors and Shine (Luster)
The most common colors of gypsum are white, colorless and clear. Rarer colors include red, brown and yellow. The luster of gypsum also ranges, from silky, dull, pearly and sub-vitreous to vitreous.
Types
Gypsum's appearance comes in a variety of types. These types include the massive variety, the fibrous variety, and the crystallized variety. The crystallized variety (also known as the selenite variety) is transparent in color, and grows in plates or in masses. The fibrous variety (also known as the satin spar variety) grows commonly as needles or prisms. The massive variety includes white and clouded types, as well as the type of gypsum known as alabaster.
Hardness and Breakage
Minerals are rated using the Mohs scale of hardness, a relative scale rated 1 through 10, where substances are tested on each other to see which is harder. On this scale a talc is rated 1 while diamond is rated 10. Gypsum rates 1.5 to 2 on the Mohs scale, meaning a fingernail can scratch it. Gypsum fractures unevenly, from conchoidal to splintery fashion, depending on the direction.
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