History and General Information of Natural Opal Gemstone

The word opal comes from a combination of the Sanskrit word ?upala? and the Latin word ?opalus?, both of which mean precious stone? The opulent flashes of color that are displayed by the opal gemstone cannot be compared to any other gemstone, making it phenomenally popular the world over. It has been regarded as the luckiest and most magical of all gems because of the special display of all colors of the rainbow.

The opal rough was first mined in Czechoslovakia (former Hungary) in the 14th century. However, in the 1800?s the gem lost its popularity since it was associated with bad luck. Like the many hues of its flashing colors, myriad legends are associated with the opal. While the Romans believed that the gem was a sign of love and hope, the Arabs believed that the gem fell from the heavens to flash its light on earth. Australian legends will have us believe that a huge opal governs the stars and the gold mines, guiding human love.

On the other hand it is also believed that the gem was actually a devil hiding in the ground with destructive flashes of wicked magic.

Opal gemstones are delicate and therefore requires careful handling. However, the sheer beauty of this gemstone outweighs this disadvantageous physical property. Certain opal gemstones are traditionally given as a gift in a well crafted  jewelry box .

The play of color makes the opal one of the favorite gemstones for jewelry. Opal jewelry like opal rings, opal earrings, opal pendants, opal wedding rings, black opal mens rings, opal engagement rings and even antique opal rings have become fairly popular. Irrespective of whether the jewelry is made of australian opals, black opals, pink opals, cherry opals or any other specific opal type, the display of colors is what makes each item look fantastic.

Composition

The chemical composition of opal gem stone is a hydrous silicate di-oxide (SiO2 + nH2O). The specific gravity ranges between 1.98 and 2.20 and the refractive index ranges from 1.44 to 1.46. The hardness of the opal on the Mohs scale is generally between 5.5 and 6. The opal is mostly made up of silica and is a very delicate stone. It also contains a significant amount of water (usually between 3 and 10 percent). In most cases, the gem is accompanied by fair amounts of cristobalite.

Varieties

Opals are classified according to the base or body color and the play of color that it displays. The five main types of opals commonly seen in gemstone trade circles are:

1) Black opal

The body color of what are known as black opals is generally black or dark grey. The black opal shows a good play of color and is the most highly priced of all the opals. The prices are even higher if the gem displays a strongly hued play of color. The black opal was first found in Australia in 1887 and therefore is sometimes also called the Australian opal.

2) White opal

The body color of this variety ranges between translucent white and medium grey. The white opal shows a good play of color and is among the most common opal varieties that can be found.

3) Crystal opal

The body color of this variety is transparent to semi-transparent. The crystal opal shows an exceptional play of color. Though it is priced well, the crystal opal is not as expensive as the black opal.

4) Water opal

The body color of this variety is transparent to translucent. It shows little or no play of color and hence is low in value.

5) Fire opal

The body color of this variety is transparent to translucent red, orange or yellow. It may or may not show any play of color. The fire opals are desired among consumers mainly due to the warm red body color that they have.

Physical Properties

Opal Color

Opals display a range of colors ranging from clear through white, gray, red, orange, yellow, green, shore, blue, magenta, rose, pink, slate, olive, brown and black. Apart from the basic body color, opals also display a unique flashing of rainbow colors; something popularly known as the ?play-of- colors?.

Cut

The cutting of the opal gem is generally done based on the cabochon cut (dome shape). Opals are not very hard and therefore they need a protective setting that can be used to add strength to these gems. A precious opal is often mounted on to a potch-opal base (usually onyx, obsidian or artificial black glass) to make the gem more durable. The mounted opal is called the doublet since it has two materials that look like one gemstone in its structure. Sometimes when the opal is very thin, it is often cut along with the mother rock and minerals that it is found in to maintain overall durability. The fire opal is sometimes fashioned as a faceted gem with a domed table.

Carat

The opal can be found in various sizes. It is mainly found in nature as thin sheets of gem material but big sizes and boulder opals are also not very uncommon.

Clarity

The opal gem can be found in the range of transparent to opaque.

Price, Cost and Buying of Opal Gemstone

The first step in evaluating an opal is determining the type of opal that it is. This is because each type has a different pricing range that it operates in. The next factor in deciding the pricing is the play of color that the gem displays. To ascertain the play of colors, the stone must be rotated completely to check for its range of colors that it throws up. A good stone will show a play of color from all angles. The color pattern and the transparency of the play of color are also noted to arrive at the price.

Red is considered the most valuable color. The rule of thumb that is followed in fine opal pricing also is that the more transparent the stone, the more its value. An opaque stone is not considered to be very valuable. Since a large part of the weight comprises of a less expensive material, the opal doublet is far lower in terms of value than a regular opal.

Markets and Producers of Opals

 Very few locations around the world have conditions that are favorable for the formation of opals.  Today, most of the opals found in the market are Australian opals and most of the opal mining is done in this continent. The continent produces almost 90% of the total global opal production. In Australia, New South Wales is the primary source of the black opal and large black opal rocks can be found in city. The fire opal is found only in Mexico and therefore the fire opal is also called the Mexican fire opal, at times. Some opals are also found in Brazil.

Treatments in Opal Gem

Opals can be treated to darken the background color and intensify the play of color. A fairly porous opal can be treated in two ways - sugar treatment or smoke treatment.

In the sugar treatment process, the opals are heated in a saturated sugar solution and then immersed in concentrated sulfuric acid. A chemical reaction takes place between the sugar and acid to produce carbon. This carbon darkens the background color of the gems. As a result the play-of-color in the stone appears brighter.

In the smoke treatment process, the opals are wrapped in silver foil paper and heated. The smoke penetrates the opal darkens the background color. This process too results in a brighter play of colors since the background is darkened.

Apart from the two treatments mentioned above, opals can also be impregnated with melted wax, resin, polymer or plastic. This process improves the durability and appearance of the gem when the filling solidifies. If the material used to impregnate the gem is colored, the gem is deemed as dyed. It is important to note that this treatment is not done on a large scale because it involves the risk of damaging the delicate gem.

Simulants of Opal Gemstone

The play of color in opals is so exclusive that baring the opalescent quartz, there is almost no natural gemstone that can be termed as a stimulant of the opal. Opal?s synthetic simulants are synthetic opal by Gilson (a company that produces synthetic gem stones), glass and plastic.

Synthetic Opal

During the 1970?s, John S. Slocum carried out various experiments to create a synthetic opal. But he could not succeed as his synthetic opals were not durable enough. They were highly porous too.

Much later, Pierre Gilson of Switzerland created the synthetic opal. Though he was able to produce synthetic opals, the complex nature of the production process and the time required for the process made these Gilson opals extremely expensive to produce. Eventually the firm sold the process to another other manufacturer, making it possible for the gem dealers to sell ?Gilson opals? at more affordable prices.

The synthetic opal is also called the honey opal due to the honey comb effect that can be seen in the play of colors. This effect is sometimes also called the lizard skin effect. Since this phenomenon can be observed only in a synthetic opal and not in a rough opal or a natural one, the property can be used to differentiate synthetic opals from natural ones. 

Interesting Trivia and other Opal facts

Legend says that the holy Roman Emperors wore crowns set with opals. These gems were believed to protect them and their families.

Reference

1) Webster
2) Gemmology by Peter G.Read
3) Handbook of Gem Identification -   Richard  T.Liddicoat, jr
4) Gems and Crystals - From the American  Museum of Natural History - Anna S.Sofianides  and George E.Harlow.