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In Simple words tt is a hydrous silicon dioxide. Alright and in simpler words it is hydrated form of silica. The distinguishable features of opal are its’ Play-of-Colour, what it’s normally known and recognized for by people. Although it comes in various colours without any POC and in various transparencies from transparent to opaque. Opal is not the usual crystalline form of silica and classified as mineraloid with approximate water content of 2-20% at times! So yes, over time it has been noted that opal will lose its weight, therefore it is vital when you care for opal to keep it stored on a damp piece of cloth or have a shot of water nearby just so it doesn’t craze (crack). The most interesting part for me about opal is its atomic structure that can be seen under highest magnification (in labs). The cause of POC is from the neatly stacked spherical atoms, they are tightly stacked (like if you imagine a crate of oranges and each row was the same and the one above it and so on) in a tight way. The common opal (also called “potch”) which doesn’t display play-of-colour will have randomly placed silica spheres.
Clockwise from Left to Right:
John Fords Opal 6.90 Carats "Lucky 7"- Black opal from Lightning Ridge
John Ford Ring: John's Connoisseur Collection- featuring Black opal from Lightning Ridge
Boulder opal in CAD design by Gems to Jewels
Rough Opal lot from Tucson show 2019
John Ford: The Lightning Ridge Collection (various opals from crystal to semi black and black set in 18 Karat white gold)
Various opals from Mintabie, Lightning Ridge and Coober Pedy available at Gems to Jewels
Thank you for reading and hope this was interesting as I thought it was when I studied Gemology!