The very earliest spectacle lenses were made of quartz crystal and were given the name "pebble lenses" in the optical trade.
http://www.visionsourceoftexarkana.com/products/glasses_reso...

4. optics crystal lens: a lens made from colorless rock crystal
Definition:

thick and distorting: being or containing lenses that make the eyes of the wearer seem very large and distorted ( informal )
wearing thick pebble glasses
http://encarta.msn.com/dictionary_1861724481/pebble.html