During dry periods, much of the water evaporated, leaving solid deposits of silica in the cracks and between the layers of underground sedimentary rock. The silica deposits formed opal.
How opal forms
Opal is known for its unique display of flashing rainbow colours called play-of-colour. There are two broad classes of opal: precious and common. Precious opal displays play-of-colour, common opal does not.
Play-of-colour occurs in precious opal because it’s made up of sub-microscopic spheres stacked in a grid-like pattern—like layers of ping-pong balls in a box. As the light waves travel between the spheres, the waves diffract, or bend. As they bend, they break up into the colours of the rainbow, called spectral colours. Play-of-colour is the result.
The colour you see varies with the sizes of the spheres. Spheres that are approximately 0.1 micron (one ten-millionth of a metre) in diameter produce violet. Spheres about 0.2 microns in size produce red. Sizes in between produce the remaining rainbow colours.
Although experts divide gem opals into many different categories, five of the main types are:
- White or light opal: Translucent to semi-translucent, with play-of-colour against a white or light grey background colour, called bodycolour.
- Black opal: Translucent to opaque, with play-of-colour against a black or other dark background.
- Fire opal: Transparent to translucent, with brown, yellow, orange or red bodycolour. This material—which often doesn’t show play-of-colour—is also known as “Mexican opal”.
- Boulder opal: Translucent to opaque, with play-of-colour against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.
- Crystal or water opal: Transparent to semi-transparent, with a clear background. This type shows exceptional play-of-colour.