Did you know that 95% of the world's precious opal comes from Australia?

How Opal Color is Produced
It took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three dimensional frame, sitting in a "bath" of silica solution. It is the orderliness of the spheres that separates precious opal from common opal.

Light passes through the transparent spheres in a direct line, but when it hits the 'bath' of silica, it is bent and deflected at different angles, thus producing a rainbow effect.

Deflection & Diffraction
Depending on the size of the spheres, varying colors of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the color in opal. If you move the stone, light hits the spheres from different angles and bring about a change in color. The name opal actually means "to see a change in color." The way in which colors change within a particular stone as it is rotated and tilted is called the stone's play of color.

How color is defined
The size of the spheres has a bearing on the color produced. Smaller spheres bring out the blues, from one end of the spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the color will be.

How Opal Formed
The consensus among geologists is that almost all Australian gem opal has formed by precipitation of silica from very dilute silica solutions or colloidal suspensions, derived from the deep weathering of feldspathic sedimentary rocks under the action of percolating groundwater.

This deep weathering is a chemical alteration.  The feldspar minerals are altered to kaolinite, releasing silica in an aqueous solution or suspension which may collect in traps or cavities in the rocks.  These cavities may be open fissures, interstices between particles in a conglomerate, holes left by dissolution of shells, bones, wood or inorganic minerals (such as gypsum and calcite), or hollow cores and cracks in ironstone concretions.

The ironstone concretions may be localised along the base of palaeochannels, on the downthrown side of differential compaction faults, or along basal undulations of bedding interfaces.  The silica solution or suspension may have been concentrated by evaporation through the overlying sediments, or concentration may have been effected by the action of clay beds which in many places underlie the opal-bearing horizons, as semi-permeable membranes allowing the water to pass through but retaining the silica.

Increased concentration my have caused the separation of discrete silica particles which aggregated by collision into equidimensional spheres.  When these reached a particular size, they may have undergone undisturbed settling and hardening into an ordered arrangement which formed precious opal.  More commonly, a disordered arrangement of spheres accumulated during settling forming common opal or 'potch'.  Because water is retained in the opal structure, application of heat, or in some cases merely exposure to a dry atmosphere, can cause fractures to develop.

Opaline silica is common throughout the leached profile of the Cretaceous Winton Formation , which consists of feldspathic sandstone, siltstone, and mudstone.  In some places, the matrix of the sandstone is replaced by opaline silica, which may also replace wood and gypsum.

Precious opal, though sometimes found in these modes, is most commonly associated with various forms of concretionary ironstone.

Glossary of Terms
Andamooka Matrix - A more porous opal found in the mining field of Andamooka. When it comes out of the ground it is quite pale but by treating it with a carbon dye process it eventually looks like real black opal. An Andamooka matrix opal can be an affordable alternative to a genuine black opal--but buyers beware--an honest opal dealer will tell you whether you are buying an Andamooka matrix or a genuine black; a disreputable dealer may not.

Amorphous - Shapeless. Not consisting of crystals. Non crystalline. Glass is amorphous. Sugar is crystalline.

Black opal - Opal which is found with a natural black or dark background. This background can range from pitch black to gray giving the stone are darkish appearance when seen from the top. It is the dark background which allows the brilliant colors. Black opal can be any color.

Black Crystal Opal - Is gem colors on a smoky, transparent background, some people regard this as the best type of opal ever found.

Body Color - This is the background color of the opal (not the color of the "flashes"); and is described as being from white, to gray, to black.

Boulder opal - Similar to black opal except that the foreground color is very thin and sits on a brown ironstone base. These stones at times can come with interesting "hills" and "valleys" on the surface. (meaning that the surface is often, but not always, undulating) These stones are preferred by people who are more "progressive" in their jewelry tastes. Often boulder opal has ironstone inclusions in the foreground and all sorts of odd shapes which makes them a designers delight.

Broken Flash - Part of the stone displays a flash of color at a time.

Consistency of Fire - This has to do with the stone's ability to show play of color when viewed from any angle. In other words, the fire is non-directional. The opposite, directional, indicates that fire is visible only when viewed from a certain angle, and are less valuable.

Crystal opal - An opal with a brilliant crystal appearance allowing you to look down into the stone.

Deflection - The bending of rays of light from a straight line.

Diffraction - The Breaking up of a ray of light into either a series of light and dark bands, or into colored bands of the spectrum.

Diffuse - To spread out so as to cover a larger space or surface. To scatter.

Fiery Opal - Predominantly red pay of color sometimes seen as pin fire or stardust flashes.

Floral - A flashy effect like a bunch of flowers, seen frequently in Queensland opal.

Fluorescent - A light produced by the electrical stimulation of a gas or vapor. Fluorescent lights have a similar effect on opal as a bright cloudy day--they do not properly bring out the colors in opal.

Gem/Precious Opal - Is defined by the brilliance of colors in the color bar.   If it is on flawless potch (common opal), it is considered more valuable than on a mixed layer of potch.

Hardness - Opal is 5.5 to 6.5 or harder, similar to Emerald.

Harlequin - Is surely at the top of the list and the most prized of all brilliant opals.   Such rarities portray spangles of rounded, angular to roughly square patches, presenting a harlequin appearance of interchanging colors.

Hydrate - A compound produced when certain substances chemically combine with water.

Hue/Fire Pattern - This describes the geometrical relationships of the colors. Mosaic (a "tiled" look), pinfire (lots of small/tiny spots of color), and flash (broad, sweeping, 3-dimensional color swaths that depend on the angle of viewing) are typical terms.

Hues and Intensity - The predominant and secondary hues are described for the play of color (flashes) present, along with a percentage of the stone "covered" by each color, and each color's intensity from poor to excellent. Other hues (tertiary, etc.) present may be listed if significant.

Jelly Crystal opal - A solid crystal opal that is extremely translucent, to the point of being almost transparent.

Incandescent - Glowing with heat (red or white hot) as in a light bulb which glows white hot, but produces a light that more closely simulates natural sunlight. Sunlight and incandescent lights bring out the natural colors in opal.

Matrix Boulder Opal - A porous sedimentary ironstone having minute cavities filled with precious opal.   When cabochon cut, the dark brown ironstone sparkles with vivid pin flecks of color

Mosaic Opal. The previously described opals are all naturally formed. The mosaic opal is a man-made creation. Slivers of genuine opal are assembled into an irregular tiled pattern with rivers of black potch framing each unique opal tile. Mosaic opals make stunning stones for all types of jewelry and allow the jeweler to include a wide range of colors within an affordable piece.  Now, there is a natural stone that looks very similar to mosaic opal. The natural phenomenon is called the harlequin opal. It is the rarest and most expensive opal.

Opal - Opal comes from the Latin word opalus which means to see a change in color. Chemically, opal is hydrated silica, similar to quartz. 

Specs:
Color: white to gray to black, blue, green, orange
Hardness: 5.5 to 6.5
Specific Gravity: 2.0 to 2.2
Cleavage: none
Fracture: conchoidal, splintery, brittle
Refractive Index: about 1.45
Transparency: transparent to opaque
Chemical Composition: SiO2 . nH2O

Opalescence - A play of color, similar to that of an opal.

Opal fossils - Opalized shells, crustations, sea creatures, snails, animal claws and bones, are some of the many objects that over time can opalize when buried in the unique environment of Australian clay and left undisturbed.

Opalized Wood - Because the creation of opal is a natural process, opals can sometimes be found within substances other than stone.

Opal patterns -  May be classified as follows: pinfire (small pinpoints of color), flashfire (larger irregular pattern of color), broadflash (sheets of color across large portions of the stone), rolling flashfire (sheets of color that "roll" across the stone as it is moved), and harlequin (square/angular blocks of color set closely together; rare).

Opaque - Not allowing light to pass through. The opposite of transparent.

Peacock - Resembling the tail of a peacock. Radiating from one central point and fanning out like the tail.

Picture Opals - Uniquely patterned boulder opals where an object is clearly visible. The specimen has been named "the shrimp." Picture opals are both rare and unique, making them much sought after by opal collectors.

Pin Fire - Seen as pinhead spots in one predominant color such as green.  The most outstanding being a color unit, perhaps red, radiating one way, which changes to another, perhaps green, when turned at various angles.  Some pin fire colors blend into peacock tail design either on part of the stone or from a central nucleus.  Other pin fire colors blend into fern or foliage design.

Play of Color - The way in which colors change as an opal is tilted in different directions.  Play of color in an opal, (its "flashes" or "fire") is caused by light interference patterns created by the tiny spheres that make up opal's structure. These spheres are about 0.001 millimeters in diameter!

Potch (Common Opal) - Common opal with no play of color.   Varies from black to milk white to greenish, yellowish or bluish in color.

Rainbow Opal - The colors are positioned as in a rainbow.

Rolling Flash - Brilliant colors rolling across the face of opal, some having cat's eye rolling effect.

Semi-Black opal - The semi-black opal is found on most fields. It's background color ranges from gray to near black. One of its distinguishing characteristics is an almost smoky appearance. Semi-black opals are from the same family as black opals, but they are not as dark.

Silica - (Silicon Dioxide) A hard, white or colorless substance, that in the form of quartz, enters into the composition of many rocks and is contained in sponges and certain plants. The needle in the mouth of a female mosquito is made of silica. Flint, sand, chalcedony, and opal are examples of silica in different forms.

Specimen - A chunk of opal that is either left in its natural state or given a polish or varnish to show off one aspect or feature of the stone. Opal specimens are sought after by rock collectors, who generally display them on a desk or shelf.

Spectrum - The band of colors formed when a beam of white light passes through a prism or by some other means (e.g. mist or spray, in the case of a rainbow) The full range of spectrum colors are: red, orange, yellow, green, blue, indigo, and violet.

Sphere - A round three dimensional geometric shape whose surface is equally distant at all points from the center point.

Translucent - Letting light through without being transparent.

Transparency - The "see-throughness" of the body is described (from watery to opaque).

Transparent - Easily seen through (glass like)

White opal - A solid opal with an opaque light background.

Yowah Nut -  Brilliantly colored opals nested in a nugget of ironstone.  Though the opal center is sometimes separated from its ironstone nugget, many beautiful and unusual jewelry pieces are made by keeping the nugget and its center-opal together.