Gypsum is one of the more common minerals in sedimentary environments. It is a major rock forming mineral that produces massive beds, usually from precipitation out of highly saline waters. Since it forms easily from saline water, gypsum can have many inclusions of other minerals and even trapped bubbles of air and water.
Gypsum has several variety names that are widely used in the mineral trade. "Selenite" is the colourless and transparent variety that shows a pearl like luster and has been described as having a moon like glow. The word selenite comes from the greek for Moon and means moon rock. Another variety is a compact fiberous aggregate called "satin spar" . This variety has a very satin like look that gives a play of light up and down the fiberous crystals. A fine grained massive material is called "alabaster" and is an ornamental stone used in fine carvings for centuries, even eons.
Crystals of gypsum can be extremely colourless and transparent, making a strong contrast to the most common usage in drywall. The crystals can also be quite large. Gypsum is a natural insulator, feeling warm to the touch when compared to a more ordinary rock or quartz crystal. Sheets of clear crystals can be easily peeled from a a larger specimen. Gypsum crystals can be extremely large - among the largest on the entire planet. Plaster of Paris is made by heating gypsum to about 300 degrees Fahrenheit, driving 75% of the water out of the mineral. This reaction absorbs energy, enabling a sheet of drywall to resist fire for a while. Heating further to about 350 degrees F drives out the remaining water and results in conversion to the mineral anhydrite.