Opals | Opal Types, Grading, History, Legend and Lore

Sponsors

Opals

"Made up of the glories of the most precious gems, to describe it is a matter of inexpressible difficulty: there is in it the gentler fire of the Ruby, there is the brilliant purple of the Amethyst, there is the sea-green of the Emerald, all shining together in an incredible union. Some aim at rivalling in lustre the brightest azure...of the painter's palette, others the flame of burning sulphur, or of a fire quickened by oil." ~ Pliny the Elder

Opal History and Lore

Opal is thought to have been discovered as long as 4,000 years ago, and myths and lore abound in practically all cultures. The ancient Greeks thought opal to be the tears of Zeus and prised it as highly as diamonds. They believed opal gave the gift of foresight and prophecy, which would ensure the owner success in war, business and life. The ancient Romans wore opal as a symbol of hope and purity and believed it could cure illness. In ancient India, opal was referred to as the Goddess of the Rainbow, turned to stone. Ancient Arab cultures believed opal had fallen from the sky and that the play of colour was trapped lightning. According to Arab lore, opal could make the wearer invisible. The ancient Australian aborigines, however, envisaged a more sinister origin. They thought opal to be half serpent and half devil, and that the brightly coloured fire within was an attempt to lure them into the devil's lair.

Pliny the Elder in his writings tells of a Roman senator called Nonius who, in 35 BC, owned a ring set with a particularly beautiful opal the size of a hazelnut and valued at 2,000,000 sesterces. Roman General Mark Antony decided he must have Nonius' opal, but when Nonius refused to sell, the enraged Antony banished him. Nonius fled Rome, leaving behind all his possesions, save the opal ring which was the cause of his exile.

Opal has been thought to have healing powers in many world cultures, and in the middle ages, it became known as the Opthalmius, or Eye Stone, and was thought to strenghten eyesight. Blonde maidens wore opals to protect their hair from fading or darkening.

Bad Luck?

So, how did opal get its reputation for bringing bad luck? There are many theories, but most historians point to the 1829 Sir Walter Scott novel, Anne of Geierstein. According to an article at Opals Down Under, "Having not read the third volume, the public jumped to the conclusion that the heroine has been bewitched, that her magic opal discolours when touched by holy water, and that she dies as a result. On carefully examining the texts, Si Frazier, writing in Lapidary Journal, found all three accusations false. The opal, which actually belonged to Anne's exotic grandmother, turns out to have turned pale as a warning to its owner against poisoning (which was the actual cause of her grandmother's death). Even so, this single work plunged opal prices to half in just one year and crippled the European opal market for decades."

Gemmologist George F. Kunz, author of The Curious Lore of Precious Stones, wrote, "There can be little doubt that much of the modern superstition regarding the supposed unlucky quality of the opal owes its origin to a careless reading of Sir Walter Scott's novel, 'Anne of Geierstein'. The wonderful tale... contains nothing to indicate that Scott really meant to represent opal as unlucky."

Another explanation for opal's bad luck reputation is that when Australia began producing high quality precious opal in the 1890s, opal began to rival diamond in popularity, so the diamond merchants began spreading the rumor that opals brought bad luck to the wearer. It was effective, and even today, there are those who believe it is unlucky to buy or wear one unless it is your birthstone.

However, Queen Victoria didn't buy into the notion that opals brought bad luck. She loved opals and made sure her subjects knew she placed no stock in the superstitions. Throughout her reign, she wore opals herself and gave them to her daughters as gifts. The Queen's efforts have been credited with helping opal shed its bad luck reputation and regain popularity with the public.

What is Opal?

The word opal is from the Latin opalus, probably from a Sanskrit word meaning ‘precious stone’. Chemically, opal is hydrated silica dioxide, SiO2.nH20. It is similar in makeup to quartz, but is softer and has varying amounts of water, between 2 and 20%, with precious opal generally containing 6–10%.

The invention of the electron microscope made it possible to see that opals are actually composed of tiny silica spheres. In gem quality opal, the spheres are uniform in size and shape and arranged in layers much like marbles stacked inside a cube. White light is diffracted by these layers and broken up into the colours of the spectrum, creating the characteristic play of colour which prompted Pliny's poetic description. The colour observed depends on the size and shape of the spheres and the spaces between them. Smaller spheres produce blues and violets; larger spheres produce reds and oranges. Even these "large" spheres are so small that about 6.5 million of them will fit in the space of a millimetre. The more uniform the spheres are in size and shape, the more vivid and defined the colors will be. The spheres in potch are irregular in size, shape and arrangement, so it has no play of colour.

Coober Pedy White Opal    SEM Coober Pedy Opal
Coober Pedy Opal            Electron Micrograph       Black Potch Opal    Electron Micrograph
With Red Fire Flash            Of Coober Pedy Opal                                                    Of Black Potch Opal

Types of Natural Opal Natural opal is opal which has not been treated or enhanced in any way other than by cutting and polishing. There are three types of natural opal.
	*Opal (Type 1)* is what most people think of when they think of gem opal. If it has play of colour, it is referred to as precious opal. Opal that does not have play of colour is called common opal or potch.
	Boulder Opal (Type 2) is a seam of opal still attached to the host rock, usually ironstone. Boulder opals with a clean face are the most valuable.
	Matrix Opal (Type 3) is distributed throughout the host rock, (usually quartzite, ironstone or basalt) rather than in seams. Most of this comes from Andamooka, Australia, and is often dyed.

Opal Grading

The following grading criteria are described in Opal Identification & Value by Paul B. Downing, Ph.D. and are based on the grading system developed by the Lightning Ridge Miners Association in collaboration with several Australian gem industry organizations.

Base Colour

The term base colour is a bit complicated because it actually includes three characteristics: colour, tone and clarity. Colour is the hue of the body plus the tone based on the Lightning Ridge Miners Association Tone Scale. Dark base colors are black, semi-black and grey. There are also red, orange, yellow, blue, green and brown base colors. Clarity refers to the degree of transparancy, from transparent (crystal) through transluscent (semi-crystal) to opaque.

Australian Opal Body Tone Scale

Opal Base Color Tone Chart

Brightness of Fire

The major factor in determining an opal's value is its brightness of fire, which is graded as Faint, Dim, Somewhat Bright, Bright, Quite Bright, Extremely Bright or Brilliant. The higher the brightness level, the more valuable the opal will be.

Fire Colour

Just like base colour, fire colour has three components: hue, lightness or tone, and saturation. These terms have the same meanings as when applied to coloured gemstones. Hue refers to the fire's colour. There will always be a dominant colour and usually a secondary colour. For example, a stone can have one fire colour, say, blue. If a stone has a dominant blue and secondary green, it has a blue-green fire. If it has other strong fire colours, it is a blue-green multicolour. Red fire is generally the most valued colour. Lightness or tone refers to the degree of black cast in the hue, and saturation refers to the intensity of the fire. Saturation can be Dull, Average or Vivid.

Consistency

Opals are natural materials and will never be perfectly consistent in colour, brightness, or pattern of fire, or in density of background colour. The higher the degree of consistency, the more valuable the opal.

Directionality

An opal that is consistently bright when viewed from any angle is more valuable than one that appears brighter from some angles than others. However, an opal with such strong directionality that it is unsuitable for a ring stone may make a beautiful pendant.

Cut

An opal's cut is important because it not only affects the beauty of opal, but its durability. Most opals are cut en cabochon, or without facets. Many are oval, but they can be cut to nearly any shape, or free-form. An opal should be cut thick enough to not be subject to breakage, but not excessively thick so as to be unattractive. Pay close attention to cut when purchasing loose opals. A stone that is too thick or too thin may be difficult to set properly. The surface should be smooth and all edges should be beveled or rounded to protect the stone from chipping during setting.

Inclusions

Opal inclusions are graded differently than those in other gemstones. Inclusions that don't detract from the beauty of the stone are frequently ignored. Cracks and crazing, however, severely affect value, because they affect both the beauty and durability of the stone.

Weight

Most opals are sold on a price per carat basis like other gemstones, but boulder opal is generally priced by the piece since the ironstone matrix is so heavy.

Locations

The source of Nonius' opal was Europe, (the Czeck Republic and Slovakia today) which was the source of all European opals in until the mid 19th century when opal was discovered in Australia. Today, more than 95% of the world's precious opal comes from Australia, and opal is Australia's national gemstone. White, or 'Milky' opal, is found in the white opal fields around Coober Pedy, Mintabie and Andamooka in South Australia. The most highly valued black opal is found in Lightning Ridge and White Cliffs in New South Wales, and boulder opal is mined in Queensland, in Quilpie, Winton, Opalton and Yowah.

The Virgin Valley in Nevada is one of the United State's primary opal deposits. Other US sources are Oregon, Idaho, Arizona, and Louisiana. Mexico is known for its orange crystal opal, called fire opal. Brazil produces crystal opal with lovely and unusual pastel fire colours. Honduras is a source of matrix opal, which forms in black basalt. Ethiopian opal is mostly orange crystal in a light or dark brown matrix often called chocolate opal.

Lightning Ridge Black Opal Mintabie Opal Nevada Opal Specimen Mexican Fire Opal Brazilian Crystal Opal Ethiopian Chocolate Opal

Lightning Ridge Mintabie Nevada Mexico Brazil Ethiopia

Assembled Opals

Type 1 opal can be sliced very thinly and made into doublets or triplets, which, when well made, offer the beauty of solid opal at a more affordable price. Doublets are backed by a black matrix material, as are triplets, which also have a cap of clear quartz or other material. The layers are easily visible when viewed from the side. Doublets and triplets are graded the same as solid opal. Small pieces of opal are sometimes fitted together and sold as mosaic opal. All of these should be disclosed by the seller.

Opal solid, doublet, triplet

Treated Opal

Because natural opal a soft stone, it is ocasionally treated with oil, plastic or wax as a stabilizer. Black Opal is occasionally treated with chemicals, smoke, or dye. A light coloured matrix opal from Amdamooka is sometimes treated with sugar and sulphuric acid to darken the body tone and accentuate the play of colour. This treated material is called Andamooka Treated Matrix Opal, and a demonstration of this process can be found at Opulent Opals. All treatments should be disclosed by the seller, and pieces should be priced to reflect the fact that they have been treated.

Synthetic Opal

Synthetic opal is a relatively new material, first successfully produced by Pierre Gilson in France in 1974. There is debate in the gemological world about the term "synthetic opal" because a synthetic gemstone by definition has the same physical, optical and chemical properties as its natural counterpart, and, though having a similar structure to that of precious opal, they lack the water content. Some synthetic opals are impregnated with up 20% polymer resin, which, combined with the lack of water content makes synthetic opal more durable and less prone to cracking than natural opal.

The Chatham company makes a "Created Opal" that is generally thought to be the highest quality lab created opal. There is also a new material on the market called Lison which claims to be closer to the natural stone because it contains water and is not polymer impregnated.

Synthetic opal can be very difficult to identify without magnification, but there are several indicators that an opal might be synthetic. They tend to have brighter, larger, more orderly colour patches that, under magnification, often show as columns of colour that extend through the stone. Under higher magnification, synthetic opal shows a cellular structure that often resembles a snakeskin or chicken wire pattern.

Opal Simulants

Opal simulants can be made of a variety of materials. One popular simulant is Slocum Stone, which is actually a material made up of bits of coloured foil and clear plastic to simulate play of colour. It is easily distinguished with a 10x loupe. Other imitations may be foil-backed glass or plastics.

Opal Care

When properly set in a mounting that protects it, natural opal can last indefinitely. However, it is soft and somewhat porous and should not be cleaned with harsh chemicals. For more complete information, visit our Jewellery Care page.

-By Cynthia B. Reuschel RGA

Share |

2/4/2012 5:44:53 PM Contact | Terms & Conditions | Privacy Policy | About Us | Fees