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Opals possess a rare beauty. They are the only gemstone to show all the colours of the rainbow. How is this possible however? What is it about opals that makes them shine the way they do?
Opals are a unique example (among gemstones) of beauty arising out of chaos. For it is not the perfection of the structure of opals that make them shine the way they do. In fact, it is the slightly chaotic makeup at the microscopic level of these precious stones that gives them their brilliance.
Opals are made up of the same materials as quartz. Both are forms of silica. However, while quartz generally forms by crystallising into ordered patterns out of molten magma, opals are formed quite differently.
Opals form from a silicon and water solution that deposits and slowly builds up in small cracks and faults underground. As the water evaporates, layers of microscopic silicon beads are left behind, closely packed into an irregular lattice.
This lattice of irregularly sized and packed silicon beads is what opals are made out of.
And this is what gives them their beauty. Due to the irregular diameter and structure of these beads, opals refract light in the same way rainbows do. When light enters an opal, it bounces around the somewhat chaotic weave and splits apart, reflecting back out of the stone as many different colours.
A small amount of water is left behind from this process. Most opals have a water content between 3 and 21% by weight. And, while most Australian opals are non-porous, many opals around the world will absorb water.
If this occurs, the porous opal will lose some of its colour or become transparent. If this happens, the opal must be dried to recover its brilliance.
Black Opal
Australia is famous for its black opals. When black opal was first discovered, a rumor was spread around that they were not actually genuine. This was believed at first because no-one had ever seen the intense display of colour seen in Australian opals ever before.
Black opals have only found in 2 places, Lightning Ridge NSW and Mintabie South Australia. Although most of the world’s black opals have come from Lightning Ridge.
Black opals appear black due to the trace elements of iron oxide and carbon that were present during their formation. The black background colour is why they shine so brilliantly. It provides contrast. One could liken it to the dark sky behind fireworks, making the show that much more spectacular.