Selenite is a varietal name of Gypsum, which is one of the more common minerals in sedimentary environments. It's primarily composed of Hydrated Calcium Sulphate, it forms when limestone is attacked by sulphuric acid.The resulting product is larger than its components so can be 'squirted' through pores in the rock under pressure, whilst in crystalline form. 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.