Wikipedia indicates that the Visby lenses are a collection of lens-shaped manufactured objects made of rock crystal (quartz) found in several Viking graves on the island of Gotland, Sweden, dating from the 11th or 12th century. Some were in silver mounts with filigree, the mounting covering the back of the lens, and were probably used as jewelry.

Visby Lens

Some of the lenses can be seen at the Fornsal historical museum in Visby, while some are in the Swedish National Museum in Stockholm, and others have been lost.

The Visby lenses provide evidence that sophisticated lens-making techniques were being used by craftsmen over 1,000 years ago, at a time when researchers had only just begun to explore the laws of refraction.

Excavations at Fröjel on Gotland in 1999 discovered evidence of local manufacture of beads and lenses from rock crystal, with unworked pieces of crystal coexisting with partially finished beads and lenses. The Vikings of Gotland were known to have participated in trade networks that reached as far as Constantinople.