Opal is an amorphous form of silica related to quartz. It is a mineraloid and not a mineral since water forms 3% to 21% by weight. Opal is the national gemstone of Australia, which produces 97% of the world’s supply. South Australia alone meets 80% of the world’s demands.

Opal diffracts light due to its internal structure. It can take on many colors depending on the conditions in which it formed. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black.

Of these hues, the reds against black are the most rare, whereas white and greens are the most common. Its optical density varies from opaque to semi-transparent. While used as a gemstone, thin layers of opal are placed on basalt – a darker underlying stone, enhancing the natural colors of the opal.

Boulder Opals are the second most valuable opals after the black opals. Boulder Opal is unique to Queensland Australia. It occurs in deposits in weathered sedimentary Cretaceous rocks in the west of the state. It is found in siliceous ironstone concretions or boulders ranging in size from less than a few centimeters to more than 20 cm.

Major boulder opal mines are at Winton, Quilpie, Yowah, Koroit and Jundah. Concretions up to 5 cm across are known as ‘nuts’. These may host a kernel of solid opal or contain a network of thin veins of opal through the ironstone.

The opal veins are tricky to follow. Many cutters split the boulder with a saw to expose spectacular colors of the rainbow. Sometimes it looks like both pieces have been polished. Such splits are used in making earrings since it is very hard to get 2 pieces of opal matching in color.

Ironstone from Koroit has very little color but have interesting patterns in black and white. Some of the brightest colors are found in Yowah nuts. Quilpie and Winton have some beautiful opals that have full face of color. These thin color bars need good cutters with experience.