Antique spectacles had different lenses than today's. Glass and plastic lenses are so prevalent in our own day, and are manufactured to such a high degree of precision, that they have completely supplanted other materials that were once used to provide optic lenses to the nearsighted. Antique spectacles frequently made use of crystal in place of glass, ground down to form lenses in much the same way as glass.
Rock crystal and glass lenses coexisted for centuries side by side, with each one having its role to play in correcting vision. Rock crystal was referred to as “pebble” in Colonial America, with much of it imported from Brazil, where quartz with exceptional clarity and excellent optical qualities was mined. “Pebble” lenses or “pebble glasses” were used widely in the pre-Revolutionary and Revolutionary period as well as the first few decades of the United States' existence.
Rock crystal continued to be used for the lenses of antique spectacles throughout the 19th century and into the first decade or two of the 20th century as well. Their use declined slowly over time as glass continued to become better refined and cheaper. The advent of mass manufacturing techniques made it possible to produce high quality glass lenses in gigantic quantities, while crystal lenses lagged further and further behind in manufacturing potential.
If you find a pair of antique spectacles with rock crystal lenses, then it almost certainly dates to a period before the 1920s. Although it is not always easy to detect rock crystal lenses at first glance, it is well worth making the effort, since this is one of several important factors used to identify a pair of glasses and their provenance.
Determining if lenses in antique spectacles are glass or crystal
Crystal lenses made out of “pebble” were created by slicing the transparent rock thin using an iron saw coated in oil and diamond dust, since metal alone was insufficient to cut the quartz. Crystal is both harder than glass and was usually ground thinner to offset its greater weight.
Glass possesses superior optical qualities, but is not as tough as crystal, so “pebbles” were used for rough and tumble situations where vision correction was required, while glass lenses were preferred for more peaceful venues where the risk of damage was low. Rock crystal lenses are indeed tougher and harder than glass, and are much more likely to survive without cracking or breaking to the present day.
Interestingly, the later antique spectacles – those from the latter end of the 19th century – often have the frames inscribed with the word “pebble” to distinguish them from glass. Check the frames carefully for any mention of this word, as this will establish both probable lens material and period.
If you can remove the lenses from the eyerims without damaging either, the edge of crystal lenses is hard enough to be able to cut glass. Additionally, to the experienced eye, the marks of cutting with a rotating iron saw and diamond dust are often visible even after so many years have passed – a testament to the skill and patience of a long-ago lenses grinder who created the centerpiece of this eyewear.