In response to a question from Epecho
Rock crystal is synonymous with clear and colorless quartz. These days, it is most used in ornamental carvings, and best known as the substance out of which crystal balls are made. Although rock crystal is common, it's difficult to find it of the size and clarity needed to make something as large as a crystal ball. (Source: Rock Crystal entry
in the Amethyst Galleries
Eyeglasses were first made in the mid-13th century, out of rock crystal, although lenses had been used to help improve eyesight since at least a few hundred years earlier. While there is evidence for lenses from Greek, Roman, and Babylonian sources (the last being archaeological evidence), the only known use for them was filtering light or for burning holes into things, or cauterizing wounds. The University of Tennesee hosts a decent summary of the history of spectacles
, although there are many, many others available online in addition to this one.
According to the OED, the first known use of the phrase "rock crystal" in English was in the 1666 Transactions of the Royal Philosophical Society
(I.362), wherein a writer grumbled that "Rock-Crystal is not fit for Optick-Glasses." Although glass, replacing rock crystal, did indeed become the cheap and widespread material from which lenses were ground, certainly by the mid-15th century, it was not until the end of the 18th century that glass was made especially for the making of glasses. Up until that point, the glass ground into lenses was taken from byproducts of other parts of the glass-making industry.