How is opal formed

A type of opal from Mexico, known as Mexican water opal, is also a colorless opal that shows either a bluish or golden internal shine. Not all fire opals are similar. We distinguish between the common fire opals, which are either faced or cut into cabochons depending on their quality, and the particularly valuable ones, which, in addition to their vivacious color, also have the typical opal gaudy color play.

But the fire opal plays its part as a high-quality gemstone to perfection with or without color play. Fire opal has a Mohs hardness of 5. Fire opal is also poor in tenacity, meaning it can be chipped or broken quickly. Fire opal is best used in accessories such as earrings, pins and pendants that are not normally exposed to rough wear.

If a fire opal is placed into a circle, it is recommended to have a setting built specifically to protect the stone from abrasion and impact. It is known as a mineraloid because of its amorphous nature, unlike crystalline types of silica, classified as minerals. It is deposited at a relatively low temperature and can occur in nearly any rock fissures, most commonly found in limonite, sandstone, rhyolite, marl, and basalt. There are two broad opal classes: precious and normal.

Play-of-color iridescence precious opal shows, not regular opal. Opal may be clear, translucent or opaque depending on the conditions under which it was made, and the background color may be white, black or almost any color of the visual spectrum.

For nearly years, Mexico has been the primary fire opal source in the world. Small fire opal pebbles can be found in silica-rich lava flows. Smaller quantities of fire opal are produced in Australia, Brazil, Honduras and Guatemala.

Some beautiful fire opals are produced in the United States, Nevada and Oregon. Fire opal is a mineral opal variety, red, orange or yellow in colour.

The visibility and clarity differ, as well as the particular complexity and brightness of its color, and these are some of the factors that determine the quality of fire opal.

This cannot be confused with the reflections caused by foils within simulant opals, as play-of-colour is prismatic or rainbow-like. Basil Anderson sums up the identification of precious opal quite wonderfully with this quote, included in The Opal Book by Frank Leechman: "As regards to identification, there is little that need be said concerning opal, since, except in the variety of fire opal, it is a stone that cannot effectively be imitated, as soon as prismatic colours are seen.

A cushion-cut fire opal. This quote still stands today. Red, orange and yellow are the most desired spectral colour to be seen in opal and therefore command higher prices.

Blue and green are less desirable colours, although still beautiful. This is the most ubiquitous variety of opal. It has a white background and is sub-transparent to translucent and usually displays opalescence. The best examples will show all the spectral colours. It is now considered to be less valuable than black opal. However fine white opal still commands high prices and is very attractive.

Displaying a black body colour with little to no opalescence, this variety is now the most desirable. An opal cabochon with flashes of orage, red and green. Once again the intensity and fullness of the play-of-colour contributes to the pricing. These varieties occur when opal forms in narrower veins and is cut and polished within the host rock. Boulder opal was first discovered in Queensland, Australia, it can also be found in Brazil and Canada.

Boulder opal veins are larger than matrix opal veins which form as a fine network within the host rock. Matrix opal is found near Yowah in Queensland, Australia.

This is the colourless variety of precious opal and can have a soft appearance. Usually the play-of-colour appears to be inside the stone against a colourless transparent background, which means it can appear minimal. Comprised of two parts glued together, the top is a slice of opal on a dark opaque backing made of plastic or another gem material. In the more convincing examples ironstone is used in an attempt to mimic the host rock of boulder opal. The opal occurs as a filling or lining between the concentric layers or in radial or random cracks in the ironstone, or as a kernel in smaller concretions or nuts.

Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock ferruginous sandstone or ironstone. Rare seam or band opal is also found and is typically encased in ironstone.

Pipe opal occurs in pipe-like structures which may be up to several centimetres in diameter within the sandstone and these structures may be hollow or opal-filled.

Wood opal is occasionally found replacing woody tissue material. As opposed to other sedimentary precious opal, boulder opal is attached to the ironstone, and stones are usually cut with the natural ironstone backing intact.

Solid opals may be cut from the ironstone material where the opal is of sufficient thickness. Boulder opals are fashioned to standard shapes and sizes but are also cut in freeform shapes to highlight their individual beauty and to avoid wastage.

Magnificent picture stones are also cut but these are mainly of interest to collectors rather than for jewellery use. Queensland Boulder Opal Association.

A Simple Explanation Opal is formed from a solution of silicon dioxide and water. A Detailed Explanation Occasionally, when conditions are ideal, spheres of silica, contained in silica-rich solutions in the earth form and settle under gravity in a void to form layers of silica spheres. The formation of Boulder Opal The Boulder opal found in Queensland forms in a slightly different method to other types of opal, forming inside an ironstone concretion.