Joseph Smith Papers description of Seer Stones and seeing things not visible to the natural eye.
"Seer Stone," Glossary, The Joseph Smith Papers website, accessed July 14, 2021
According to a European tradition of folk belief reaching back at least into the middle ages, quartz crystals or other stones could be used to find missing objects or to see other things not visible to the natural eye.
Scholarly background on history of seer stones.