Eye Agate

Eye agate is agate (banded chalcedony, a microcrystalline form of quartz, SiO₂) with a concentric ring pattern that looks like an eye; Mindat lists it as a variety of agate. In the Japanese bead trade the name 天眼石 (tengan-seki) usually means black agate with white eye-like bands, and this black color is commonly produced by dye: a laboratory report published by the retailer Pascle identified its sample as natural polycrystalline quartz (black onyx) colored with dye.