Opal

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 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 Mythology

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.

Source: Gem Select