A quartz mineral valued for its iridescence and showing variations in colour from white to black and in transparency from transparent to opaque. Numerous types are known, but usually, only the transparent and translucent varieties are used as gems. Opals contain between 1% and 21% water. The name derives from the Latin word Opalus meaning “stone” or “jewel” (14th century). The ancients believed that the storm god, jealous of the beauty of the rainbow god, broke the rainbow and that pieces of the rainbow falling to the earth became a part of the opal. In spite of the superstition that it brings bad luck, opal has been used as a gemstone for many centuries. In the Middle Ages, blonde maidens valued opal necklaces as they believed they helped their hair to keep its colour. Opals were also believed to help women to relax during childbirth. The native American Indians and the Australian Aborigines have used opal to invoke visions and it is known as a “stone of happy dreams and changes”. it is thought to disperse infections and alleviate fever, purify the blood and the kidneys, regulate insulin production and improve eyesight. Opal is one of the birthstones for October (with tourmaline), symbolizing hope.