Opals

On the nature of opals

Ethiopian opal from GIA.

Ethiopian opal from GIA.

The magical opal gemstone is composed of hydrated silica.  Few people realize that opals contain water and can actually be 3 - 21% water. The most common range falls between 6 and 10%.  Opals are generally formed when silica gel seeps into crevices in rocks -- and occasionally into other items. As the water evaporates, the silica is deposited in the form of tiny spheres.

The internal structure -- those silica spheres and their arrangement -- of gemstone-quality opal diffracts light.  Depending on the conditions in which the opal formed, it can take on many colors. Precious opal are found in a bewildering range of color: from clear to white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black.

Mexican fire opals.  From the National Gem Collection at the Smithsonian Institution.

Mexican fire opals.  From the National Gem Collection at the Smithsonian Institution.

In almost every publication, you will find different methods of categorizing opals.  The Gemological Association of America, however, is the world's largest and most respected nonprofit institute of gemological research and learning.  We'll rely on them.  Here's what they have to say:

The five main types of opal

Black opal. Image from the Gemological Institute of America.

Black opal. Image from the Gemological Institute of America.

  • White or light opal: Translucent to semi-translucent, with play-of-color against a white or light gray background color, called body color.
  • Black opal: Translucent to opaque, with play-of-color against a black or other dark background.
  • Fire opal: Transparent to translucent, with brown, yellow, orange, or red body color. Often doesn’t show play-of-color.
  • Boulder opal: Translucent to opaque, with play-of-color against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.
  • Crystal or water opal: Transparent to semitransparent, with a clear background. Shows exceptional play-of-color.

On finding opals and what to look for

Throughout time and up to the end of the 19th century, the best opal was mined in the andesite lava flows in what is now Slovakia. Then, in the 1890s, the incredible opal deposits of Australia were discovered. They are currently considered the best and most important sources for opal gemstones.  But opal is found in many other places which yield their own astonishing stones: Brazil, Guatemala, Honduras, Indonesia, Japan, Mexico, Russia, Ethiopia, and the United States.

Opal from Idaho.  Smithsonian Institution.

Opal from Idaho.  Smithsonian Institution.

There are three main factors that determine an opal’s quality:

  • Color
  • Pattern
  • Clarity
Crystal opal.  Image from the Gemological Institute of America.

Crystal opal.  Image from the Gemological Institute of America.

Color.  With a gemstone like opal, color is a complicated conversation.  Let's begin with bodycolor.  Bodycolor is the background color of an opal and some colors are generally more coveted than others. Although it is always a matter of taste, buyers tend to place more value on opals with a black bodycolor, which tends to display play-of-color best.
 
Play-of-color is the feature that make opals seem magical, as though they contain a unique personality of their own. The term play-of-color describes the way colors change as a gemstone is rotated and tilted. In an opal, it could be any hue found in a rainbow.  Some opals might display one color.  Some opals might display two.  Some three.  Some seem to joyfully contain every color all at once!  Play-of-color must be vibrant to bring the opal high marks.

Other color factors affecting an opal's value include the range of the play-of-color, the hue of the play-of-color (red is traditionally best, followed by orange, then green), and the color change when viewed from different angles.

Pattern. Pattern describes the organization of an opal’s play-of-color. Some of the fascinating examples of pattern include:

  • Pinfire or pinpoint: Small, closely set patches of color
  • Harlequin or mosaic: Broad, angular, closely set patches of color
  • Flame: Sweeping reddish bands or streaks that shoot across the stone
  • Peacock: Mainly blue and green
The harlequin pattern in this black opal has a lot of red, blue, and green play-of-color. From GIA.

The harlequin pattern in this black opal has a lot of red, blue, and green play-of-color. From GIA.

Opals with large, closely arranged patterns of color are rare and more valuable than those with tiny, scattered dots.

A free-form black opal from Australia. This Art Nouveau pendant was made by Tiffany and is on display in the Minerals and Gems Gallery at the National Museum of Natural History.

A free-form black opal from Australia. This Art Nouveau pendant was made by Tiffany and is on display in the Minerals and Gems Gallery at the National Museum of Natural History.

Clarity. In an opal, clarity refers to the degree of transparency and absence of inclusions. An opal can range from transparent to opaque. In a crystal opal transparency is desirable, but a black opal should be opaque. It all comes down to which type of background best displays the opal's play-of-color. A cloudy or milky background color lowers the value of any opal.

Other characteristics that affect an opal's clarity, and therefore its value, include pits, fractures, blemishes, crazing, and fragments of the host rock.

And as with other gemstones, cut and carat are also important factors when evaluating a stone. A lapidary will take an opal's color, pattern, and clarity into consideration when determining how best to cut the stone. Free-form shapes are common among high quality opals and often are the best way to exhibit their exceptional play-of-color.

Historical notes and other interesting facts about opals

In 35 B.C. the Roman senator, Nonius, went into exile rather than sell his treasured opal ring to Mark Antony.  In those days, opal was a rarity in Roman parts.  This mystical gemstone was only found in the mines of Czernowitza, in a land now known as Slovakia.  The Romans, although they came close, never quite conquered this region.

The word opal comes from the Greek language, meaning changing color.

The Dark Jubilee Opal is a 318.4-carat free-form polished black opal from a mine in Coober Pedy, Australia. From the Smithsonian Institution.

The Dark Jubilee Opal is a 318.4-carat free-form polished black opal from a mine in Coober Pedy, Australia. From the Smithsonian Institution.

Because of their water content, opals can be detected by dowsing, a technique for searching for underground water, minerals, and so on, by observing the movement of a pointer (traditionally a forked stick, now often paired bent wires) or the changes in the direction of a pendulum, supposedly in response to unseen influences.

The coveted red fire in opal gemstones is a result of larger spheres of silica inside the stones.  The more common blue fire is caused by smaller spheres.

The Goths believed that opals were made from the eyes of heaven.

Pink opal is a variety of common opal from Peru.  From the Smithsonian Institution.

Pink opal is a variety of common opal from Peru.  From the Smithsonian Institution.

Thunderstones, so called by Arabic scholars, were rose opals that would bring their wearers good fortune and health.

The Orphanus was the name of an opal found in the crown of the Holy Roman Emperor in the thirteenth century.  Set in the very center of the crown, it was said to be white as snow with flashes of red like wine.

Napoleon gave Josephine an opal called "The Burning of Troy."  It was red and weighed almost 140 grams.

In Sir Walter Scott's ghost story Anne of Geierstein, a baroness has a mysterious relationship with the opal she always wears.  It reflects all her moods with little sparks and tongues of flame.  One day a drop of holy water falls on the stone and it "shot out a brilliant spark like a falling star." We won't spoil the rest of the story here but suffice to say that many consider this story the origin of the opal's reputation for bad luck.

That said, there was a belief in Northern Europe that the magic found within opals was created from the eyes of murdered children -- a sign of the evil eye.

The Southern Cross Mine in Australia yields the famous “Yowah Nut,” precious opal in ironstone nodules or concretions. From the Smithsonian Institution.

The Southern Cross Mine in Australia yields the famous “Yowah Nut,” precious opal in ironstone nodules or concretions. From the Smithsonian Institution.

In Coober Pedy, one of Australia's most famous mining towns, hundreds of people live in caves they have dug underground.  You can stay at an underground hotel, shop in underground stores, and, yes, even attend a variety of churches... underground.

Hang around opals for long enough, and sooner or later you will come across doublets and triplets.  Doublets are thin veneers of opal adhered to darkened glass, obsidian, or colorless potch (opal with no color).  Triplets have three layers: the top is glass or quartz, the middle is a thin slice of opal, and the bottom is black glass, potch, etc.  These composites can have a great deal of beauty but they are obviously less valuable.  A dealer is required by law to inform his customers that they are buying a doublet or a triplet -- and not a solid opal.

Precious opal replacing calcite of bivalve shells, from Coober Pedy, display specimen, South Australian Museum.  Image from Wikipedia.

Precious opal replacing calcite of bivalve shells, from Coober Pedy, display specimen, South Australian Museum.  Image from Wikipedia.

Sometimes an opal is not just a gemstone -- but a fossil.  Opals can be found in the shape of fossilized materials like wood, shells, and bones.  A dinosaur skeleton on display at the National Museum in Sydney sparkles from its display case -- its bones at some point in time turned to opal.

Resources and further reading

The Gemological Institute of America's Gem Encyclopedia.

Gemstones of the World. By Walter Schuman.  Our go to book at Grimballs.

Jewels: A Secret History. By Victoria Finlay.  An non-fiction adventure through time and place exploring the history of gems.

Smithsonian Institution National Museum of Natural History's Gem Gallery.