Amazing Opals - All About October's Birthstone

The creator came down to Earth on a rainbow in order to bring a message of peace to all the humans. At the very spot where the creator's foot touched the ground, the stones became alive and started sparkling in all the colors of the rainbow. That was the birth of the opals.

-Ancient Aborigine Legend

For ages opals have been believed to have healing power, it is said to heal depression, and even help the wearer find true love. All of nature's splendor is reflected in the manifold opulence of gem-quality opals. All the colors of the rainbow, fire, lightning, and the soft shine of the sea can be seen in this seductive stone. Looking at an amazing opal is mesmerizing; it's no wonder so many legends and beliefs have formed around this enchanting stone. The best opals can command prices per carat that rival the most expensive diamonds, rubies, and emeralds.

The name “opal” is derived from the Sanskrit “upala,” meaning valuable stone. This was probably the root of the Greek “opallios,” meaning color change. The first appearance of references to opal in Roman history was around 250 BC, a time when opal was valued above all other gems. In Roman antiquity, there existed a so-called “opalus” or “stone of several elements.” So Romans, it seems, may have had an inkling as to why opals show such a striking play of colors.

Opal is hydros silicon oxide. It's a mineraloid, a mineral-like substance that doesn't demonstrate crystallinity. It's the national gemstone of Australia, which produces 97% of the world's opals, mostly coming from the dry and remote Outback deserts. Parts of Australia were one covered by a vast inland sea millions of years ago. Stone sediment was deposited along the shoreline. When water masses flooded back, they flushed water containing silica into the resulting cavities and niches in sedimentary rocks and also remains of plants and animals were deposited there. Slowly the silica stone transformed into opal. Basically, opals are a combination of silica and water. Or, more precisely, opals are a gel from silica, with varying percentages of water. An opal can contain from 3% to 21% total weight in water, but usually only 2-6%. Although they come mostly from Australia, they are found all over the world, and even on other planets. In 2008 NASA Mars Reconnaissance Orbiter discovered a number of opal deposits on Mars, another piece of evidence that once water existed on the planet. One of the most important factors in determining the quality of an opal is what es perts call the “color play” or “fire.” This aspect of the opal was largely a mystery until the 1960's, when a team of Australian scientists put opals under an electron microscope and discovered that small spheres of silica, 150-300nm in diameter, cause interference and refraction of light, resulting in this play of colors. These spheres dissect the light in its passage through the stone and turn it into all the colors of the rainbow, always new and different. The size of the spheres and their geometric packing determine the color and quality of the diffracted light. There are endless different kinds of opals, and many different ways to grade them, and the experts have many words to describe different types of color play.

The basic types of opal:

Precious Opals

This is a general classification for the opals which flash iridescent colors when viewed from different angles, or play of color. This is what makes opal popular and its desirability is based on the intensity and diversity of the color, uniformity, and ability to be seen from different angles. These opals are very rare.

Common Opals

These opals do not exhibit play of color. They are given the name “common” because they are found in many locations all over the world. The are also common in appearance, with one uniform, opaque color throughout the stone. But still, they can be cut and polished to create an attractive piece at a reasonable price. Peruvian blue and pink opals are a well-known example of this type, and both exhibit delightful color.

Fire Opals

These opals also lack play of color, and are known for their bright, fiery, transparent or translucent color throughout the stone. Usually they are orange or red, but can also be found in bright yellow. They are one of the very few types of opals that are faceted, although translucent fire opals are often cut en cabochon, a more traditional cut for opals. Querétaro, Mexico is famous for fire opals.

Opals can also be differentiated by their relationship to the host rock. Solid opals consist entirely of opal material without any host rock or other significant inclusions. Boulder opals have grown within the host rock, forming in the voids and fractures of other rocks. Boulder opals reveal this aspect of opals in their finished form; they are the rarest type of opal at only 2% of the world's production. Matrix opals are precious opals with an intimate mix with the parent rock instead of being confined to seams and patches like Boulder.

Opals can also be classified based on their base color. The most common color is white. It can also be referred to as light or milky opal. It is the least expensive, and the most common of the precious opal colors, accounting for 60%.

Black opal is the most desirable color because of the contrast between the color of the fire and the color of the body. The dark body makes the fire effect more obvious. It is very rare, at only 8% of the world's opal production, mostly coming from Lightening Ridge in Australia. Honduras is also well-known for its black opal.

The second most popular or desirable color is crystal opal, which is transparent or translucent with play of color within the stone. It accounts for 30% of the world's opal production.

Blue, pink, and morado opal are all common opals, displaying no play of color, but showing a single color throughout the stone. Blue and Pink are often from Peru, and morado (Spanish: purple) from Mexico.

Since the play of light in an opal is so important, experts have innumerable ways to refer to the different fire patterns. Here are just a few:

Harlequin – patches of fire in the shape of rectangles or diamonds

Contra-Luz – color play is only visible when a light source is behind the stone (must be transparent or nearly transparent)

Pin fire – pinpoints of fire throughout the stone

Cat's Eye – optical effect similar to cat's eye, with a thin line of fire visible from multiple directions, tracking back and forth across the stone

Opals are also often classified by their place of origin.

White Cliffs – the first opal mining by prospectors was done in this field in Australia in 1890. The spot is famous for pineapple opals or opal Pseudo-morphs – rare opal in various fossils.

Andamooka – another early mining district of South Australia, starting in the 1920's. The area is famous for matrix, crystal , and light opals.

Coober Pedy – a small town in South Australia, settled in 1916, when opal mining began. The name is Aborigine and literally means “white man in a hole,” which is how the early prospectors made a home in this sweltering desert. With a pick ax and a bucket these men created a home in a hole to protect themselves from the heat of the day and the icy wind of night, hoping to find opals in the dirt they removed along the way. The place is known as the opal capital of the world, famous mostly for white base-color opals.

Honduras – known for its black opal with matrix or pin fire color play.

Peru – mostly blue and pink common opal.

Louisiana – mines in Vernon Parish are known for quartzite cemented with precious opal.

Most cut opals are solid, meaning the entire stone is cut from a single piece of rough opal. Some opal has a very thin layer of fire that is quite brilliant. The artisan cuts the stone down to this thin layer and then glues it to a base of obsidian, potch, or basalt, then shapes it into a finished stone. A 2-part stone like this is called a doublet, and in some settings can be impossible to differentiate from a solid opal. Sometimes, to protect the thin layer of soft opal, and accentuate the fire, a crystal clear top layer of quartz, spinel, or other transparent material is glued onto the opal – this makes it into an opal triplet, or three-part stone.

How to tell a fake:

Synthetic opal or lab-created, have been around since the 1930's. They are made with the same materials as real opals (hydrated silicon dioxide) and some look very similar to genuine opals. They are most easily recognized by the fire pattern that resembles snippets of foil in glossy matrix of streaks and spots of fire with a geometric shape.

Gilson Opal or Gilsonite was created by Pierre Gilson in 1974 after the discovery of the ordered sphere structure of natural opal. They are made of lab-created material with a chemical composition different from natural opal. These do not fluoresce under UV light, which is the surefire way to tell them from genuine opals.

Imitation opals are made of plastic or another glassy substance – not silicon dioxide. Usually they have a pearly opalescence rather than a genuine play of color. Often they're called “Opalite” when sold in stores.

Caring for your opal:

Opal is softer than most gemstones, at 5.5-6 on the MOH's hardness scale. For this reason it's ideal to set an opal in a piece that rarely gets scuffed, like earrings or a pendant. For a ring it's ideal to set the opal inside of a protective setting like a bezel cup that doesn't expose the edges. It is poetically beautiful that the best care for an opal is to wear it often. An opal is made partly of water, which can evaporate if not properly cared for, causing the opal to be brittle and appear dull. It used to be necessary to occasionally oil unworn opals for this reason, but more recently many opals have been sealed with an artificial, colorless resin.