A form of silica (SiO2), often occurring as
stalactites and found in many types of rock.
Opal is cryptocrystalline, that is, the
crystals are too fine to be detected under a
microscope. The common opal is translucent,
milk-white, yellow, red, blue, or green, and
lustrous. Precious opal is opalescent, the
characteristic play of colours being caused
by close-packed silica spheres diffracting
light rays within the stone. Opals are found
in Hungary, New South Wales, Australia (black
opals were first discovered there in 1905),
and Mexico (red fire opals).