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Opal Gemstone Information

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About Opal - History and Introduction

Opal is a gem-quality form of hydrated amorphous silicon dioxide. Its name is derived from the Sanskrit word for 'stone'. It is gemologically classed as a mineraloid rather than a mineral, owing to its amorphous form. Opal is considered to be the national gemstone for Australia, owing to the fact that Australia produces roughly 97% of the world's entire supply of opal gemstones. Opals can be divided into three main subgroups: precious opal, fire opal and common opal (potch).

Opal is famed for its ability to diffract light. The exact cause of opal's unique properties was only recently discovered by Australian scientists in the 1960s after analysis with electron microscopes. It was discovered that small spheres of silica gel caused interference, refraction and diffraction of light, resulting in opal's distinctive play of color. The varying refractive indices of the spheres and spaces between them dissect the light on its passage through the stone. As light enters the opal, it bends around the tiny particles or 'spheres' of hydrated silica, as well as 'chips' of silicon and oxygen suspended within the stone. Light is comprised of all visible colors and can produce an entire spectrum of colors when it is diffracted.

Precious opal is known for its remarkable ability to diffract light, which results in rainbow-like colors that change with the angle of observation - known as 'play of color'. Fire opal can sometimes exhibit slight color play, but it is better known for its vivid body color. Common opal is usually opaque, rarely translucent, and lacks play of color. It is often found mixed with other gemstones, such as agate opal or moss opal. Common opal is known to exhibit 'opalescence'. The term 'opalescence' is often mistaken for 'play of color'. Opalescence should technically only be used to describe the optical effects seen in common opal. Opalescence is caused by the reflection of light and appears as a sheen of light, typically milky-bluish in color. It is a form of adularescence, whereas 'play of color' is iridescence caused by light diffraction.

Opal Gemstone

Identifying Opal

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Opal by definition is hydrated silicon dioxide, always containing from three to thirty percent water. This characteristic and its relative softness with a rating of 5.5 to 6.5 makes opal quite easy to identify and distinguish from other gemstones. Unlike other gems with play of color or iridescent effects, opal also exhibits 'opalescence'. Other iridescent stones do not display this pearl-like bluish color effect that appears to glide across opal, but instead, will usually show color that will abruptly appear, disappear and then reappear, depending on the viewing angle. In most cases, close observance of color and optical phenomena can usually help distinguish opal from other similar gemstones.

Opal; Origin and Sources

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The most important deposits are located in Australia. Around 95 percent of the world's supply of white precious opals are mined "down under". Famous deposits in New South Wales are at Lightning Ridge and White Cliffs, in South Australia at Coober Pedy and Andamooka. Various deposits are also found in Queensland.

Other deposits are found in Brazil, Ethiopia, Guatemala, Honduras, Indonesia, Japan, Mexico, Russia and the United States (Idaho and Nevada).

Buying Opal and Determining Opal Value

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Opal Color

Opal can occur in many colors including white, colorless, pale yellow, pale red, gray, brown and black. Diffraction can cause flashes of any color of the rainbow which can be seen in opal's color play. Yellow, blue and green are most common, whereas violet, red and orange are the rarest colors seen through play of color. Opalescence is usually milky blue in color, but it can occur in rarer colors. The saturation of body color, the play of color and the transparency determine an opal's value.

Opal Clarity and Luster

Opal gemstones can occur transparent to opaque. Transparent to translucent materials are typically more valuable than opaque materials. Almost all opal will have some visible internal fractures or inclusions such as patches, potch (common opal) or matrix rock inclusions. Opal tends to exhibit a waxy to resinous luster which helps accentuate play of color and opalescence. Finer stones often have a vitreous luster.

Opal Cut and Shape

Opals are usually cut into shapes which maximize and preserve the weight of the rough opal. They are often cut en cabochon and depending on the opal color bar, it may be cut with low or high domes in order to maximize play of color and adularescence. Opal can also be faceted, most often seen with 'fire opal'. Opals can be found in many shapes, including traditional ovals, rounds and cushions, as well as fancy hearts, marquises and trillions. Opal is often carved into ornaments.

Opal Treatment

Opal is typically untreated, but it may be impregnated with oil, wax, or plastic to enhance color and stability. It may also be layered (as in opal doublets or triplets) for durability. 'Smoking' treatments are now more common than before. 'Smoking' of opal is done to darken body color which enhances play of color, most often seen with Ethiopian opal. There are forms of synthesized opal available. Technically they are 'imitation opal' as they often include substances like plastic, which are not found in natural opal.

Opal Gemological Properties:

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Chemical Formula: SiO2_nH2O - Hydrous silicon dioxide
Crystal Structure: Amorphous; kidney or grape-shaped aggregates
Color: All colors, partial play of color
Hardness: 5.5 to 6.5 on the Mohs scale
Refractive Index: 1.37 to 1.52
Density: 1.98 to 2.50
Cleavage: None
Transparency: Transparent to opaque
Double Refraction or Birefringence: None
Luster: Waxy to resinous
Fluorescence: White opal: white, bluish, brownish, greenish; fire opal: greenish to brown

Please refer to our Gemstone Glossary for details of gemology-related terms.

Opal: Related or Similar Gemstones

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Chocolate Opal
Chocolate Opal

Opal is hydrated silicon dioxide, which means it has an extremely high water content and an amorphous crystal structure. There are many different varieties of opal with some being extremely rare and valuable. In addition, there are also several unofficial 'marketing' names used for opals based on specific localities, inclusions and other properties. Some of the official and most well-known opal varieties, include rare black opal, chocolate opal, fire opal, boulder opal, opal doublet, opal in matrix, common opal and precious opal.

The following lists some of the most well-known opal trade names, with a short description:

  • Agate opal - agate with light and dark opal layers.
  • Angel skin opal - misleading name for palygorskite, an opaque, whitish to pink-colored silicate mineral.
  • Black crystal opal - transparent to semi-transparent opal, with a dark body color and play of color.
  • Black opal - precious opal with near-black to black body color. The rarest and most valuable of all opal.
  • Boulder opal - a thin seam of precious opal on ironstone matrix. Since this is a natural occurrence, its value is higher than that of a man made doublet. The ironstone is very dark, which makes the fire stand out and gives a close resemblance to black opal.
  • Chocolate opal - precious opal with a brown body color. Also one of the rarest and most valuable opals available.
  • Common opal, also called potch, is opaque, rarely translucent and shows no play of color. A wide variety of trade names are used to identify common opals.
  • Crystal opal - transparent to semi-transparent, colorless body with play of color.
  • Fire opal - translucent to transparent, with a yellow, orange or red body color. May or may not have play of color. Also called Mexican opal or sun opal.

  • Girasol - almost colorless, transparent opal with bluish opalescence.
  • Harlequin opal - transparent to translucent precious opal with mosaic-like color patterns. Counted among the most desirable opals.
  • Honey opal - honey-yellow translucent opal.
  • Hyalite - glass opal or water stone, colorless, water-clear opal with strong sheen.
  • Hydrophane - a milk opal, which turns turbid due to loss of water. It can become translucent again and have color play through absorption of water.
  • Jelly opal - bluish-gray precious opal with little play of color.
  • Moss opal - milk opal with dendrites.
  • Opal in Matrix - opal in matrix is similar to boulder opal. Precious opal is dispersed in small fragments throughout the matrix rock, rather than in a seam or even layer.
  • Opal doublets - a thin layer of precious opal glued to a black base.
  • Opal triplets - a thin layer of precious opal glued to a black base with a transparent cap composed of quartz or other similar material, such as synthetic spinel or sapphire.
  • Pink opal - pink opal is delicate, but some opal, such as Peruvian opal, form together with chalcedony, which results in a slightly harder and more durable opal.
  • Porcelain opal - white, opaque milk opal.
  • Prase opal - apple-green opal.
  • Precious opal - displays rainbow-like hues that change with the angle of observation, especially in rounded cut forms.
  • Semi black or gray opal - precious opal with a dark body color.
  • Wax opal - yellow-brown opal with wax-like luster.
  • White opal - precious opal with white or very light body color.
  • Wood opal - yellowish or brownish opal in the form of fossilized wood.

Opal Mythology, Metaphysical and Crystal Healing

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The name opal was probably derived from Sanskrit "upala", meaning "valuable stone" or from the Greek word "opallus" which means to see a change in color. Later, the Latin word "opalus" evolved, meaning precious stone. In the days of Roman antiquity there existed a so-called "opalus", or a "stone from several elements". Pliny the Elder, the famous Roman author, called opal a gemstone which combines the best possible characteristics of the most beautiful of gemstones; the fine sparkle of almandine, the shining purple of amethyst, the golden yellow of topaz, and the deep blue of sapphire, "so that all colors shine and sparkle together in a beautiful combination". According to an old Australian Aboriginal legend, the Creator came down from the heavens on a rainbow and delivered a message of peace for all mankind. Stones that were near the place where His feet touched the ground suddenly came to life and began to sparkle. This was believed to be the birth of opal.

For ages people have believed in the healing power of opal. It is reported to be able to solve depression and to help its wearer find true and real love. Opal is also said to stimulate originality and creativity. Opal is porous and because of this, it is quite absorbent. Due to its ability to absorb, it is thought that it can pick up the thoughts and feelings of people and amplify emotions. Opal is a birthstone for those who are born in October.

Disclaimer: Metaphysical and Alternative Crystal Healing Powers and Properties are not to be taken as confirmed advice. Traditional, Ceremonial and Mythological Gemstone Lore is collected from various resources and does not represent the sole opinion of SETT Co., Ltd. This information is not to replace the advice of your doctor. Should you have any medical conditions, please see a licensed medical practitioner. GemSelect does not guarantee any claims or statements of healing or astrological birthstone powers and cannot be held liable under any circumstances.

Opal Gemstone and Jewelry Design Ideas

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Opals are best suited for earrings, brooches and pendants. Due to the delicate nature of opals, they are not recommended to be worn daily as rings or other types of jewelry that are subject to hard knocks, although most still choose to wear opal rings, just with extra care to prevent damage to the stone while wearing. Opals can be drilled to make attractive pendants for necklaces or earrings. They can also be bezel set or wire-wrapped for occasional-wear rings, bracelets, necklaces and pendants. Tumbled opals also make eye-catching beads. Opals are also often carved in ornamental designs, such as animal or flower carvings. Opal gemstones are suitable for both men and women, and large cabochon opals are ideal for oversized gemstone rings or pendants. Black opal, chocolate opal, boulder opal and opal in matrix are all very popular for use in men's fashion accessories.

Note: Buy colored gemstones by size and not by carat weight. Colored stones vary in size-to-weight ratio. Some stones are larger and others are smaller than diamonds by weight in comparison.

Famous Opal Gemstones

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'Big Ben' is a white precious opal of over 800 grams, more than 4000 carats, which was found at Lightning Ridge, Australia.

'The Light of the World' is a white precious opal of 40 grams, 2,250 carats, which was found in Australia. It is considered to be the most colorful white opal of all time.

The 'Andamooka Desert Flame', is the largest opal ever brought to the light of day; weighing 6,843 kilograms.

Opal Gemstone Jewelry Care and Cleaning

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How to clean your gemstonesOpals are delicate gemstones. Their most significant weakness has to do with their water content. If an opal is allowed to dry, it will crack and fade. If you live in a very dry climate, or keep them in a dehumidified room, some precautions are necessary. Keeping them in a tight plastic bag, with a damp piece of cotton or fabric will prevent dehydration. Because of their water content, opals are also highly sensitive to sudden changes in temperature.

Being somewhat soft, opals scratch easily. It is important to realize that a large component of ordinary dust is quartz at 7 (Mohs scale) in hardness. At 5.5 to 6 in hardness, simply wiping the dust off an opal will gradually reduce its polish. The solution is to clean your opals using a soft cloth and only mild soap or detergent if needed. Do not use harsh chemical or cleaners and avoid ultrasonic cleaners and steamers. Always remove opal jewelry before engaging in vigorous physical activity.

  • First Published: December-15-2006
  • Last Updated: January-17-2019
  • © 2005-2019 GemSelect.com all rights reserved.
    Reproduction (text or graphics) without the express written consent of GemSelect.com (SETT Company Ltd.) is strictly prohibited.
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Size and Weight

Gems are always measured in Millimeter (mm)

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length x width x depth,
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