Quartz crystals.

The term 'crystal' originally meant quartz, which occurs in fine well-formed mases of such striking beauty that it attracted attention from early times. It was the earliest crystalline substance to be studied closely. Some coloured varieties, such as amethyst, are used as gemstones. Quartz crystallises in two forms which are mirror images of each other. It played a part in the development of the theories of stereochemistry which originated in the optical studies of Biot and Pasteur in the mid 19th century.