Grey crystal opal from Lightning Ridge, Australia. It shows green-blue play of colour on a light grey base. It has even level of play of colour at every angle, a slight advantage for jewellery design purposes.
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It all happens deep within the earth, when the hot water with a lot of silica dissolved in it, begins to cool. As its temperature drops, it cannot hold the silica anymore and starts to deposit it. If the conditions are right, that silica will then settle in the form of microscopic spheres. Hopefully uniform in size and arrangement across a small patch. When a lot of those patches, made of different sized silica spheres, solidify into one mass. The precious opal is born.
The size of those silica spheres and the gaps between them is what decides what colour you will ultimately see, coming out of any given patch within the opal. When the lights passes through, larger gaps allow longer light wavelengths to be reflected towards the viewer, while smaller gaps will result in shorter wavelengths. The shortest wavelength we can see is violet and the longest one is red. It is no wonder then, that red is the rarest colour in most of opals, as it requires larger spheres to be formed. As always, there are exceptions and in this case, it seems that red is far more common in Ethiopian opals than it is in Australian ones.
Basic division is between precious and common, with the latter not possessing any play of colour. Moving past that, into the precious opal category, we divide opal by its body tone. It can vary from completely black, through all shades of grey, into white. There are also transparent stones, called crystal opals. In general the darker the body tone the higher the price premium. It is because a darker body tone provides a better contrast for the play of colour to shine against. Also because almost all black opal comes for a single source- Lightning Ridge, Australia. A quality crystal opal can circumvent those rules, especially when it shows multiple layers of fire, creating a mesmerising 3-D colour show.
Following that, it is important what colours are present and what kind of pattern do they form. There’s a whole nomenclature of pattern names used in opal trade but in the end, you simply must see the stone. Opal is just too diverse to rely on a description alone. Usually the larger and more geometrically uniform the patterns are, the more desired the gem is. Equally important is the total amount of play of colour and the intensity of it. The origin is also an important factor for the price. There are many opal sources in the world, starting with the leader- Australia. It provides over 90% of total yearly supply at the moment. Other sources include Mexico, Brazil and a recent newcomer- Ethiopia.
Opals hardness varies between 5 and 6.5 on a Mohs Scale. While wearing opal you should generally avoid impacts, abrasion and immersion in liquids other than water ( household chemicals, oils, etc.). To clean your opal, it’s best to use a wet cloth or gently clean it under the running water. You should avoid ultrasonic cleaners.