Gypsum (CaSO₄·2H₂O) is a common mineral in sedimentary environments. The crystal structure of this calcium sulfate dihydrate can be defined as alternating double-sheet layers of sulfates bound covalently by calcium ions and single-sheet layers of water molecules linked by weak hydrogen bonds. The mineral gypsum has several varieties that differ by the shape of the crystals and their textural arrangements. The variety called gypsum alabaster is made of fine-grained crystals and is used for craftworks. The satin spar variety is made of tiny fibrous crystals and has an attractive silky luster. The most famous variety of gypsum is called selenite, which is characterized by colorless and transparent crystals.

Large selenite crystals were very valuable in Roman times because they were used for covering windows, in therma (baths) and palaces. Roman stonemasons took advantage of a well-known physical property of transparent selenite crystals, namely cleavage, the ease with which some crystals split along definite planes where the atoms are weakly bonded to the adjacent layers of atoms, thus creating smooth surfaces. In the case of gypsum, that plane is pinacoid (010), which corresponds to the plane parallel to the layers of molecules in the crystal structure.

According to Pliny the Elder, the largest, high-quality selenite crystals were found in Segobriga, central Spain. This was the main source of crystals for window coverings until the introduction of the flat glass technology in the Roman Empire at the end of first century A.D. Amazing as these crystals were, they cannot be compared in size or quality with the crystal wonderland recently discovered in Naica, a mining town located 112 km (70 mi) SE of Chihuahua City in Northern Mexico.