What is Opal? Opal is a hydrated amorphous form of silica. Because of its amorphous character, it is classed as a mineraloid. Unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock. Being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.
There are two broad classes of opal: precious and common. Precious opal displays play-of-color common opal does not. Play-of-color is defined as “a pseudo chromatic optical effect” resulting in flashes of colored light from certain minerals, as they are turned in white light.”
Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque. The background color may be white, black. Or nearly any color of the visual spectrum. Black opal is considered to be the rarest, whereas white, gray. And green are the most common.
shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. At microscopic scales, precious opal is composed of silica spheres some 150–300 nanometres in diameter in a hexagonal or cubic close-packed lattice.
It was shown by J. V. Sanders in the mid-1960s that these ordered silica spheres produce the internal colors, by causing the interference and diffraction of light passing through the microstructure of the opal.
The term opalescence is commonly used to describe this unique and beautiful phenomenon. Which in gemology is termed play of color. In gemology, opalescence is applied to the hazy-milky-turbid sheen of common or potch opal which does not show a play of color.
Besides the gemstone varieties that show a play of color, the other kinds of common opal include:
-The milk opal, milky bluish to greenish (which can sometimes be of gemstone quality).
-Resin opal, which is honey-yellow with a resinous luster
-Wood opal, which is caused by the replacement of the organic material in wood with opal
-Menilite, which is brown or grey.
-Hyalite, a colorless glass-clear opal sometimes called Muller’s glass
-Geyserite, also called siliceous sinter, deposited around hot springs or geysers.
-Diatomaceous earth, the accumulations of diatom shells
-Common opal often displays a hazy-milky-turbid sheen from within the stone.
Fire opal is a transparent to translucent opal. With warm body colors of yellow to orange to red. Although it does not usually show any play of color, occasionally a stone will exhibit bright green flashes. The most famous source of fire opals is the state of Querétaro in Mexico. These opals are commonly called Mexican fire opals.
Fire opals that do not show a play of color are sometimes referred to as jelly opals.
Opals of all varieties have been synthesized experimentally and commercially.
Synthetic opal has been in the marketplace since the 1970s. Many of the early synthetic opals could easily be separated from natural opal, with a quick examination unaided by magnification. However, the manufacturers of synthetic opals have been improving the appearance of their products. Today many of them are more difficult to detect. Features that a synthetic opal might exhibit to reveal a laboratory-grown origin include:
1) the play-of-color patches might display a columnar growth pattern when viewed perpendicular to the growth direction
2) under magnification, a synthetic opal’s play-of-color areas might exhibit a “chicken wire” or “snake skin” pattern
3) resin-impregnated synthetic opal often has a lower specific gravity than natural opal
4) play-of-color patches are often more uniform in size and distribution across the face of a synthetic opal
5) synthetic opals are sometimes stained an outrageous color. Or the stain produces absorption bands when viewed through a spectroscope.
Using the features above, much synthetic opal can be confidently separated from natural opal. But some synthetic opals can be challenging to identify.
Synthetic opals can exhibit a spectacular play-of-color appearance that often exceeds the beauty of many natural precious opals. They are produced in a wide range of colors and patterns that many people enjoy.
Many synthetic opals look so much like natural opal that trained gemologists can have difficulty separating them from natural opals. This is why whenever synthetic opals are advertised or presented for sale. Sellers are required by law to clearly communicate that they are manufactured by people and they are not natural opals.
The main reason that people produce synthetic opal is a hope of being able to manufacture it at scale and at a price that is lower than natural opal. They have been incredibly successful. Many types of synthetic opal are now cut into beautiful ring-size cabochons that sell for just a few dollars each. Even the very best synthetic opal can be cut into cabochons. And sold for just a small fraction of the price of natural opal cabochons of similar size and appearance.
Synthetic opal certainly wins some buyers away from natural opal, but it is not likely to displace natural opal from the gem and jewelry market. Why? Most people who love opal are glad to pay a higher price to own a gemstone that formed within the Earth.
