Gypsum is a very soft hydrated calcium sulfate mineral that forms from the evaporation of seawater or groundwater bearing dissolved ions from evaporite rocks and replenishing of water, occuring in settings such as lagoons, hot springs, or caves. The evaporite mineral most commonly occurs in two varieties: selenite, the clear crystal form, and alabaster, its chalky, white massive form, and is usually associated with other evaporite and carbonate minerals, such as anhydrite, halite, sylvite, calcite, and dolomite. Its massive form typically accumulate as layers associated with sedimentary deposits, such as shale, limestone, dolostone, or rock salt, and can occur on smaller-scales, as exhibited in the image, or can form to be larger-scaled sedimentary gypsum beds. Due to its varying forms and depositonal environments, gypsum can have a vitreous to sugary lustre and can vary in color from clear to white to yellow, red, or brown from impurities. However, its most diagnostic properties are its low specific gravity of 2.3 and hardness of 2, being softer than a fingernail. Historically, gypsum has been used for plaster and ornamental purposes, being used for construction, medicinal casts or dental molds, sculptures. It's also commonly used in products like fertilizers, toothpaste, and cement.