LIGHT SOLID, WHITE  or CRYSTAL OPALS

Main location where it is found:  Coober Pedy, Andamooka and Mintabie in South Australia.

Light opal is a solid opal that is most often cut into overlays or        doublets however good solid stones or natural doublets are also common.  This opal is similar to black opal, however rather than having a black background, there is a range of background colors ranging from clear to white to light blue.  There is also a new variety called "semi-black" opal that has a grey background.  Having beautiful color at a much more affordable price than black opals, this is one of the most popular precious opals. 

Light or Crystal Opal is divided into two categories:  the crystal variety which is translucent and the milky opal which is opaque.  Both can be gem quality and used for jewelry. 

A full range of fire colors, flashes and radiant swirls can be found in the depths of a light opal.  Light crystal opal is translucent and often shows amazingly sharp colors.  When clear and colorless, this form is referred to as 'water' or 'jelly' opal.

BOULDER OPALS

Main location where it is found:  Queensland, Australia

Boulder opal occurs in the boulder opal belt, an area between the New South Wales border and northwest Queensland. Boulder opal forms on a dark ironstone base (the host rock) and occurs as a thin uneven layer adhering to the ironstone. Because of the uneven layers, sometimes part of the ironstone is visible on the surface of the stone

Boulder opal occurs in two main forms, 'seam' and 'matrix'.

When opal intrudes into cracks and seams in the host rock (usually ironstone),
it is called SEAM boulder opal.

Seam boulder opal is often 'split' along the seam so as to produce one or two faces of opal material

BLACK OPALS

Main location where it is found:   Lightning Ridge, Australia

Australian Black opals are the most valuable and widely known type of opal.  Black opal is characterized by a dark body tone which can range from dark grey to jet black.  However this refers only to the general body tone of the stone, and is not related to the rainbow or spectral colors present in the opal. Some people expect a black opal to be completely black (in which case it would be completely worthless).

Unlike ordinary opals, black opals have carbon and iron oxide trace elements present, which cause the unusual darkness of the stone. Because of their dark body tone, the rainbow colors in a black opal stand out much better than lighter opals.

This vibrancy of color makes black opal the most valuable form of all opals. Often a black opal will have a natural layer of potch (colorless opal) left on the back of the stone, which can give the stone an extra darkness and vibrancy of color. The darker this potch backing is, the darker the body tone, which usually makes for greater vibrancy of color and a higher value black opal. This is the principle upon which doublets and triplets are based, which are an imitation of solid black opals.
 

COMMON OPAL

Main location where it is found:  Many locals around the world

'Common opal' is classified as non-gem quality opal. There are several varieties of common opal; most are opaque and none exhibit any 'play of color'.   'Hyalite', or 'Mullers Glass', is a colorless opal that gives the appearance of glass.  Rarely, it does display a faint tint of color (blue, green or yellow).  'Hydrophane' is an opaque porous opal that becomes transparent when immersed in water.  'Resin opal' is black or brown with a resinous lustre.  'Potch' is generally opaque and can be milky white, pale to dark grey, bluish grey or black.  'Magpie potch' is made up of black and white patches.  A clear amber variety of potch has been found at Lightning Ridge.

 

OPALISED FOSSILS

Wood, mussels, clams, snails, ammonites, worm burrows, fish skeletons and even dinosaur bones can be opalized. 

The Australian opal fields were at one time under the sea, so opalised fossils are occasionally unearthed. As the ages passed and the seas began to recede, millions of sea creatures were isolated and marooned.  Eventually the area dried completely and the inland is now a dry desert country.  In time the ground waters, holding silica solution, also evaporated. They left behind the phenomenon known as 'opal'. 

SYNTHETIC, LAB CREATED / GROWN OPALS

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. These lab grown opals are chemically identical to natural opals.  The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974 (Klein and Hurlbut, 1985, p.528). The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous; some may even stick to the tongue.
 

IMITATION OPALS

Most so-called man made opals are more correctly termed imitations, as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewelry are often "Slocum Stone" consisting of laminated glass with bits of foil interspersed. 

MOSAIC OPALS

Mosaic opals are usually made up of many irregular shaped pieces of precious opal that is fitted together into a jigsaw-puzzle like arrangement by these fitted pieces being glued to a backing.  They can be very pretty, however they are a little more fragile than solid opals, overlays or doublets. 

DOUBLET or OVERLAY OPALS

Opal doublets are man-made.  A piece of solid precious transparent or translucent colored opal is glued to a dark backing (can be ironstone such as Zanzibar's cutters use or can be potch, plastic or another material), imitating nature's solid stone. The dark base enhances the opal's color and adds strength and thickness. 

Doublets are cut with the finished show of flash/color in mind, thus the opaque layers of opal are cut off, leaving the truest colors, brightest flash and best quality of the opal to be viewed.  In many cases, doublets are far showier and impressive than solid opals because of this.  Doublets can be quite valuable, but are also a very cost-effective way of possessing beautiful opal on a budget.  Doublets should never be submerged in water.

 

SOLID OPALS & NATURAL DOUBLETS

Solid opals are, well, just that - solid globs of opal without any matrix or other materials attached or interspersed.  They opals are typical of Australian black opals, however light (white) opals as well as others can also be solid opals or with thick enough veins, even boulder opal can be cut into solids.  While solids are impervious to water damage as doublets are, they often have flaws and or show less flash and color unless they are exceptional (and expensive) stones.  Some of the opals that Zanzibar sells are solid opals or natural doublets. 

Solid opals can also be called "Natural Doublets" when the opal is naturally still attached to the dark colored potch and or ironstone.  Often small segments of this matrix material can be seen in the finished jewelry as the edge is not usually perfect. 

TRIPLET OPALS

Triplets are also man made.  Essentially, a micro-thin cut piece of opal is sandwiched between a clear quartz or class dome (which protects the opal) and a dark backing - usually plastic but could be natural potch or ironstone.  Not of very good quality, the only real use of these is as rings as the quartz dome helps protect the opal.