Selenite (mineral) was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 tons, is long, and is over 500,000 years old. Gypsum occurs on every continent and is the most common of all the sulfate minerals. Gypsum is formed as an evaporative mineral, frequently found in alkaline lake muds, clay beds, evaporated seas, salt flats, salt springs, and caves. It is frequently found in conjunction with other minerals such as, copper ores, sulfur and sulfides, silver, iron ores, coal, calcite, dolomite, limestone, and opal. Gypsum has been dated to almost every geologic age since the
Mineral spring water. Historical mineral springs were often outfitted with elaborate stone-works — including artificial pools, retaining walls, colonnades and roofs — sometimes in the form of fanciful "Greek temples", gazebos or pagodas. Others were entirely enclosed within spring houses. For many centuries, in Europe, North America and elsewhere, commercial proponents of mineral springs classified them according to the chemical composition of the water produced and according to the medicinal benefits supposedly accruing from each: Types of sedimentary rock - usually limestone (calcium carbonate) - are sometimes formed by the evaporation, or rapid precipitation, of mineral spring water, especially at the mouths