Opals rough guide and worth
There's a huge variety of opals in the world, depending usually on the colours, frailty and iridescence patterns, exactly like the 4 C's for Diamonds (Opals-Quality Factors of the Gemological Institute of America-GIA). The opal is composed of silica and water: when the water evaporates and precipitates at low temperatures to form layers or nodules of opal in fissures, veins, and cavities of volcanic and sedimentary rocks, it generates some silica deposits; it's a procedure that protracts during millennia as at the same time is very simple.
Opals, when found, are not so beautiful and colourful: they are just pieces of stone covered with concretion and excavation soil , but at the time of cutting them, the beauty and the preciousness comes out. Stay tuned for the Black Opal follow-up on next blogs.
Unearthed raw Opals
The gem is quite particular and delicate, low humidity can cause the loss of it's iridescence (polishing may solve the problem) it’s sensitive to extreme temperatures and malleable to work being almost as hard as glass. Because of it's value , the difficulty is on losing as less material as possible during cutting and shaping, maximising control over polishing and cutting errors (cracks, burns etc.)
Opal is considered a unique gem but there are many varieties: the White Opal is the most common one, it’s quite opaque, but veins are really colorful; the Crystal Opal is very bright and coloured like a rainbow, it usually has a translucent base; the Semi-black Opal has a grey base, translucent or opaque and it’s really sparkling; and then the Black Opal is the most precious one, with vivid colors and iridescences, from green to violet, red and gold.
Nowadays the most expensive opal is the "Virgin Rainbow" which is valued over $1 million and was purchased by the South Australian Museum in Adelaide which will be presenting it as part of their "Opals" exhibit to celebrate 100 years of gemstone mining in the region.