Opal

The Opal (Q, "Inwissar") is a mineraloid gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, and basalt.In Middle-Earth the white, yellow or red, Yellow-green (catseye), Black Opal (Black Grey to black) and fire opal or Naurond) were known.