chalcedony

noun

1. (countable, uncountable)A form of fine-grained quartz that is nearly transparent or has a milky translucence; it fractures conchoidally.

   “Although there are dozens of different types of gems, among the best known and most important are […] . (Common gem materials not addressed in this article include amber, amethyst, chalcedony, garnet, lazurite, malachite, opals, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.)”