The technology for making plane glass was not invented until the first century of our era. Until then, window openings in palaces and in temples were covered with thin sheets of limestone or alabaster. In the prosperous Roman Republic, a flourishing business arose out of using strange sheets of transparent crystal found in the Iberian peninsula: Hispania glass.
In Republican Rome they did not yet know how to make sheet glass; therefore, they covered the wall openings of their palaces, baths and greenhouses with translucent rocks such as alabaster. But the most sophisticated material, the most chic, was the transparent stones that they extracted from some fields in the province of Cuenca, which were none other than crystallized gypsum: Hispania glass. These were crystals of up to a metre whose transparency and size, according to Pliny the Elder in his Natural History, made them unique in the known world.
|Selenite Crystals||Roman mines of “lapis specularis”, the Latin name for monocrystalline gypsum|
Crystal was the name the Greeks and Romans gave to any transparent and faceted mineral, whether quartz, mica or gypsum.
This gypsum was easy to separate into slabs, which were exported throughout the Roman Empire. The trading centre of Segobriga grew up out of the mining of Hispania glass, and died a natural death when, in imperial Rome at the end of the first century CE, sheet glass was created. Its production substituted the crystals from Hispania, not only ending the viability of Segobriga but also appropriating the word“crystal”for synthetic glass. Thus the confusion (particularly in Romance languages) that endures until today.
|Recreation of how sheets of Hispania glass (Lapis specularis) were made in Roman times.|
Basilica of Saint Sabina, 5th century CE. It was built on the site of the house of the Roman matron Sabina, who was later canonized as a Christian saint. Its windows are made from Selenite, not glass (taken from a nearby temple of Juno).
The word “crystal” is often used to refer to glass, especially in Romance languages like Spanish, but the glass in windows or spectacles is not crystal at all. What in English we call “crystal glass” is not crystal either, but is glass with lead or zinc added to make it look like crystal.