Premium Pebble Spectacle lenses in the 19th century were made of either optical glass or pebble. Also known also as rock crystal, some of the best pebble lenses were imported from quartz stone mined in Brazil and offered incredible clarity and hardness, which made them resistant to scratching and breaking. Cutting these lenses was a laborious process: The stone were sliced into slabs with an iron circular saw, then the curves were ground with emery, and finally, pliers or nippers were used to shape the lenses.
Premium pebble lenses could be difficult to differentiate from less expensive glass ones. With a refractive index of 1.5442 and a specific gravity of 2.7, pebble is much denser than crown glass and could be polished to a higher luster. One way to test the difference was by touching the material with the tongue-pebble seems colder. Opticians used a handheld tester that comprised two thin sheets of tourmaline that were naturally polarized and blocked light; it showed a pattern when a pebble lens was inserted. Photos, clockwise from left: handheld tester from The College of Optometrists, London, UK; pebble spectacles from the Optical Heritage Museum;
brazilian pebble ad from Joseph Bruneni’s Looking Back: An Illustrated History of American Ophthalmic Industry.