Opal

by

Dallas Weaver

www.emporia.edu/earthsci/amber/go340/students/weaver

This webpage project was created for a gemstone and gemology course at Emporia State University. The assignment was to learn webpage creation and present a summary of my knowledge of the gem known as opal.

Table of Contents


Image 1: Opal Doublet

Introduction

"The Opalus is a precious stone which hath in it the bright fiery flame of a carbuncle, the pure refulgent purple of an amethyst, and a whole sea of the emerald's spring glory, and every one of them shining with an incredible mixture, and very much pleasure" (Nicols, 1652, p. 86). This is how Nicols described the pure brilliance of opal in 1652 (Opal Historical References, 2007). Opal is an intriguing stone that possesses unique characteristics of fire and play of color. When looking at a piece of opal directly, it does not seem like anything special; but looking at it again with light reflecting off of it, all of the colors of the rainbow may be revealed in sparkling flakes, dashes, or specks (Gemstone, 2007).
Opal is derived from the word "upala" which means valuable stone. This is probably the root for the Greek term "upallios" which means color change (ICGA). It is a hydrated silicon dioxide consisting of random chains of silica (Opal, 2007).

Types of Opal


Image 2: Opal Doublet
There are three main types of opal: common opal, fire opal, and precious opal. All of these will have the same physical characteristics, but visual phenomena will vary with each type.
Common opal is rarely transparent. It may be white, gray, yellow, blue, green, or pink. It may look dendritic or have many inclusions such as moss. It is usually cabbed or used as a backing for other, more desirable gemstones. If an opal is lacking in the typical play of color, it is considered a common opal (Gemstone Gallery, 1996-2007).
Fire opal is unique in terms of opal because it usually does not possess that brilliant play of color that is typically associated with opals. It is a fiery red translucent stone that is mainly sold for its color and clarity. Unlike most opals, fire opal can usually be cut and faceted into a fine gemstone for setting into jewelry as are the specimens shown on the right (Gemstone Gallery, 1996-2007).
The most popular type of opal is, of course, precious opal. It has the most brilliant play of color of the three types and is the most expensive too. The internal play of color is classified by three things: the background color, intensity of color display, and the stone's size. Precious opal comes in all different colors. The most common colors are white, light blue, and green. Red, orange, and violet are more desirable. The rarest and most valuable of all opal is the one and only black opal. It becomes even more valuable if it contains reds and oranges. Black opal has become so highly desired that it is often priced in the same category as diamond, emerald, and ruby (Gemstone Gallery, 1996-2007).

Image 3: Fire Opal
There are dozens of varieties of opal. This website, www.minerals.net/mineral/silicate/tecto/quartz/images/opal/opalvary.htm, contains information and images of many opal varieties. The colors are evenly distributed in some varieties allowing different shades to dominate separate parts of the stone. In other varieties, there are small patches of every color spread throughout the stone. These stones are polychromatic and are known as harlequin opals (Opal Historical References, 2007). Opal variations are practically unlimited and all will have a play of color unique to the individual stone (ICGA).

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Formation


Image 4: Opal in Petrified Wood
Opal is made of small spheres of silica gel tightly packed together. The sphere dissects the light as it passes through, splitting it into many different colors. The arrangement of these silica spheres, whether it is completely random or somewhat ordered, will affect the stone's play of color quality. Opal also contains varying percentages of water within it. The higher the water percentage, the more brittle the stone will be (ICGA).
Most opal is 50-65 million years old, dating back to the Cretaceous period (Bernardine Fine Art Jewelry, 1998-2008). Opal is found in areas that were once covered by an inland sea. Water washed silica-containing solution into the cracks and crevices of sediments along the shoreline. The silica then hardened and became opal. Sometimes the remnants of plants and animals were washed up along with the silica and became part of the stone (ICGA). Opal can also become the petrifying material in things such as wood, shell, and bone, like the petrified wood shown left from the Peacock Opal Mine in Virgin Valley (Friedman, 1997-2003).
Opal may occur with many different habits such as massive, botryoidal, reniform, stalactitic, earthy, nodular, and may be found in veins, crusts and accumulating mounds. It commonly occurs with chalcedony and is often confused with it, but their difference in hardness is the distinguishing factor.

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History

Opals were relatively rare until the first half of the 19th century. It became quite popular during the art deco era because the play of color could be beautifully combined with enamel and other popular materials (ICGA). Black opals used to be artificially made by dipping the stones in black ink or heat treating them with black oil. Natural black opal was not discovered until almost a century later in the 1900's (Historical, 2007). The cause of the beautiful play of color that is unique to most opals was not determined until the 1960's. A team of scientists from Australia were the first to use an electron microscope to study the internal structure which revealed the light-splitting silica spheres (ICGA).

Legends and Superstitions

Early legends about opal trace back to the Aborigines of Australia. They believed that the creator came down to earth on a rainbow to bring the message of peace to the people of the Earth. The spot where the creator touched the ground immediately became alive and sparkling with all the colors of the rainbow (ICGA). Arabic legends say that opal falls from the sky in the form of lightning because of its brilliant flash (GIA Library, 2008).
In the 19th century, many superstitions about opal began. People believed that it must be a pleasure to own such a fine stone, therefore it must bring good fortune to the owner. Most superstitions reveal opal to be a lucky stone, but there are others that consider opal to be a stone of misfortune (Opal Historical References, 2007).
Ancient Greeks believed that opal gave the owner the gift of prophecy and guarded them from disease. Opals were considered a symbol of hope, purity, and truth to Europeans. It was even once believed to preserve the life and color of blonde hair (GIA, 2008).

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Facts

Mohs' Hardness5.5-6
ColorAll colors (play of color)
Density1.98-2.50
Cleavageabsent
Fractureconchoidal
Crystal SystemN/A amorphous
Chemical Composition SiO2O nH2O
Refractive Index1.44 - 1.47
Streakwhite
Specific Gravity2-2.5
Lustervitreous to pearly
Classmineraloids
Fluorescencewhite, blue, brown, green
Pleochroismabsent

Opal is one of the most delicate gems that are commonly worn. It has a relatively low hardness, and the inclusions of water make it light weight and very brittle. Opal is very sensitive to heat and humidity. Sudden changes to either could cause "crazing" (cracking) in the specimen, adding to its brittleness. The various colors that appear under a fluorescent light are often a good method to determine the stones authenticity, but is not always a reliable method (Opal, 1995-2007). Opal tends to look more brilliant when its in water or slightly warmed up, so dealers will often hold the stone in their hand before showing it to customers (Opal Historical References, 2007).

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Locations

Almost 95% of opal comes from Australia. Opal is mined on every continent in the world, with the most mines being in the United Sates and Europe (ICGA). According to the United States Geological Survey, Opal is mined in over half of the United States. The largest numbers of these mines are located in the western states and along the east coast, with a few spread out over the Midwest as well. The best black opal was known to have come from Australia but there have been no major finds in recent years (Gemstone, 2007).

Healing Power

There are many ailments that opal has been thought to help with or cure all together. It has been believed to aid in inner beauty, faithfulness, and eyesight, and could even help recall past lives (Bernardine, 2008). It has been reported to heal depression and help the wearer to find love. It has been used to enhance the positive characteristics of people that are born under the zodiac sign of cancer. Those born under Scorpio are recommended to wear black opal, while boulder opal is supposedly lucky for persons born under Aries (ICGA).

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References

Image References

Related Links
Emporia State University
www.emporia.edu 		Earth Science at ESU
www.emporia.edu/earthsci
Mineralogy Webpage Assignment
www.emporia.edu/earthsci/
amber/go340/webpage.htm 		Past Student Projects
www.emporia.edu/earthsci/
amber/go340/students/stupages.htm

For more information email dweaver@emporia.edu. Created April 28, 2008; last update April 29, 2008.
Return to the GO 340 Gemstone and Gemology course assignment or student webpages.