Black/dark opal shows a play of colour within or on a dark body tone, while the play of colour of a black opal is within or on a black body tone, when viewed from the face up. It can be crystal or opaque. Some black/dark opals have a light crystal colour bar on dark opal potch (colourless opal), giving the otherwise light opal a dark appearance. Even expensive black/dark opals may have only a very thin colour bar on black potch.
Most black/dark opal is found in the mines around Lightning Ridge, NSW. Because of its relative scarcity compared to light and even boulder opal, it tends to be more expensive, given equivalentcolors, clarity and patterns. Black/dark opal exhibiting bright flashes of red is extremely rare.
Boulder Opals
Boulder is a variety of precious opal which has an ironstone host rock bonded naturally to the gem. Often just a thin vein of precious opal is present. It occurs in specific locations over a wide area of Queensland where the opal fills cracks or voids in ironstone boulders. Boulder opal can be black, dark or light depending on the appearance of the stone when viewing the presentation face.
Matrix Opals
The term matrix opal is commonly used where the opal is intimately diffused as infillings of pores or holes between grains of the host rock in which it was formed. Andamooka matrix opal is a porous material from Andamooka, South Australia, which is often treated to enhance thecolor by depositing black carbon by chemical treatment in the pore spaces in the stone.
Doublets and Triplets
A doublet is composed of two sections, with a thin slice of opal comprising the top portion. Natural opal is fused to a black stone backing, leaving only the color of the opal visible
A triplet opal is so named because it has three components: a thin slice of crystal opal sandwiched between a clear top and a black backing. The top is quartz or glass while the backing can be black non gem quality opal or manmade resin.
More to come.....
See some of my opals here Boulder Opals
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