Upal. Is a precious stone having the colours of the rainbow more or less lively, more or less effulgent, and changeable according to the different positions of the stone to the light. In it are seen the red of the ruby, the purple of the amethyst, the green of the emerald, besides yellow, blue, and sometimes black and white. When the stone is broken most of these colours disappear, which shews that they arise by reflection from one or two principal ones. Its form is always either round or oval, like a pearl, its prevailing colour white. Its diversity of colours makes it almost of equal value with a sapphire or ruby. Tavernier says, there are mines of opal in Turkey. Others say Cyprus, Arabia, Egypt, Bohemia, and Hungary produce it; whence it is distinguished into two kinds, oriental and occidental. Pliny and Solinus mention a species of opal called exaconthali-thus, which had sixty colours. There is an artificial opal, counterfeited so as nearly to represent the natural stone. The opals contain silex, alumina, lime and iron.