Precious, Black, Fire, Boulder, Jelly, Crystal, White Opal
A photographic guide for the large confusion of names that are frequently used to describe opals.
What is Opal?
What is Opal?
Gem quality opal is one of the most spectacular gemstones. A single stone can flash
every color of the spectrum with an intensity and quality of color that can surpass the
"fire" of diamond. The best opals can command prices per carat that rival the most expensive
diamonds, rubies and emeralds. They are very popular gems.
Opal is a wonderful stone for earrings, pendants, brooches and rings. However, it is softer than most other
gemstones. Opal has a hardness of about 5.5 to 6.0 on the Mohs hardness scale. Because of that opal works
best in earrings, brooches and other pieces that rarely encounter skuffs and impacts. When used in a
ring the best designs have a bezel that protects the stone - instead of being placed in a prong setting that
allows the edges of the stone to be exposed.
Opal is a hydrous silicon dioxide (SiO2.nH2O).
It is amorphous, without a crystalline structure, and without a definite chemical composition.
Therefore it is a "mineraloid" rather than a "mineral".
Play of Color and Opalescence
Opal is a very common material that is found throughout the world. Most of this opal
is "common opal" or "potch" which has a milky or pearly luster known
as "opalescence". However, rare specimens of opal produce
brilliant color flashes when turned in the light. These color flashes are known as a
"play of color". Opal specimens that exhibit a play of color are known as "precious
opal." If the play of color is of high quality and large enough to cut the material can be used to produce valuable gemstones.
A play of color in opal can be observed under three situations: 1) when the stone is moved, 2) when the light source is moved,
or, 3) when the angle of observation is changed. The video at right illustrates "play of color" in
an Ethiopian Welo opal.
What Causes "Play of Color"?
Areas within an opal that produce a play of color are
made up of microscopic spheres of silica arranged in an orderly network. As the light passes through the
array spheres it is diffracted into the colors of the spectrum. The size of the spheres and their
geometric packing determine the color and quality of diffracted light.
Sources of Opal
Although opal is found throughout the world, almost all of that opal is common opal of very little value. Most of the
precious opal deposits that have been discovered are in Australia. The mines of Australia produce at least 90% of the
world's precious opal.
Famous mining areas in Australia include: Coober Pedy, Mintabie, Andamooka, Lightning Ridge, Yowah,
Koroit, Jundah and Quilpie. Other countries that produce precious and fancy varieties of common opal include:
United States, Mexico, Hungary, Indonesia, Brazil, Peru, Honduras, Guatemala, Nicaragua, Slovakia, Czech Republic and Ethiopia.
Wonderful Names Used to Describe Opal
There are many types of opal and a wide variety of names are used to communicate about them. If you have spent a small
amount of time looking at opal you have probably been surprised by this extensive vocabulary of wonderful names.
There is acually a logic behind names such as fire opal, black opal, jelly opal, boulder opal, matrix opal, Coober Pedy,
Mintabie, Andamooka, precious opal, opal doublet, and opal triplet. The stories on this webpage will present that logic
and help you see the common sense behind the names. And, since pictures are worth a thousand words we share our favorite opal photos
to help you understand. Enjoy!
"Precious opal" flashes iridescent colors when it is viewed from different angles, when the stone is moved or when the light source is moved. This phenomenon is known as a "play of color". Precious opal can flash a number of colors such as bright yellow, orange, green, blue, red or purple. Play of color is what makes opal a popular gem. The desirability of precious opal is based upon color intensity, diversity, uniformity, pattern and ability to be seen from any angle.
Precious opal is very rare and found in a limited number of locations worldwide. Most precious opal has been mined in Australia, secondary sources include: Mexico, Brazil, and the United States. Canada, Honduras, Indonesia, Zambia, Guatemala, Poland, Peru, New Zealand and Ethiopia. The black opal on the left was mined at Lighning Ridge, Australia and the white opal on the right was mined at Coober Pedy, Australia..
"Common opal" does not exhibit a "play of color". It is given the name "common" because it is found in many locations throughout the world. Most specimens of common opal are also "common" in appearance and do not attract commercial attention.
However, some specimens of common opal are attractive, colorful and lustrous. They can be cut into gemstones that accept a high polish. They can be beautiful but simply lack a play of color that would earn them the name "precious". Common opal is frequently cut as a gemstone and can command reasonable prices. Shown at right is a honey-colored opal from Mexico and teardrop-shaped stone cut from Peruvian blue opal.
"Fire Opal" is a term used for colorful, transparent to translucent opal that does not exhibit a "play of colors". Instead, it has a bright, firey color that is present throughout the stone. Fire opal is usually orange to red in color, however, many people apply the name "fire opal" to stones that are a bright yellow color.
Fire opal is cut in a variety of ways. Transparent fire opals are usually faceted and translucent stones are usually cut into cabochons. The specimen on the left is a faceted fire opal cut from material mined in Oregon. It is 9 millimeters by 7 millimeters and weighs 1.2 carats. The yellow stone on the right is a faceted fire opal cut from material mined in Nevada. It is a 9 millimeter round that weights 1.7 carats.
Opal Names: Based Upon Opal and Host Rock Relationships
Solid Opal --- (Type 1 Opal)
"Solid opal" is a name used for a rough or cut stone that consists entirely of opal material without any host rock or other significant inclusions contained within the stone. Solid opal can be a combination of precious opal and common opal. The solid opal on the left is a white opal made from material mined at Coober Pedy, Australia. The black opal on the right was made from material mined at Lightning Ridge, Australia. Solid opal is also known as "Type 1 Opal".
"Boulder opal" is a term used for a rough or a cut gemstone that displays opal within its host rock. Opal often forms within voids or fractures in its host rock and specimens of boulder opal reveal this aspect of opal's origin. The contrast of color can be striking when a bright flash of opal is seen within a the surrounding rock material. Many people enjoy the natural appearance of boulder opal and find these gemstones to be beautiful, interesting and educational. The specimen on the left is a boulder opal bead cut from material mined in Australia. It is 10 millimeters by 6 millimeters in size, and weighs about 1.9 carats. It shows numerous fractures in the parent rock that have been filled with opal material. It is a story in a stone. The stone on the right is a boulder opal from Honduras. It is unusual because the host rock is Andesite. It is about 8 x 13 millimeters in size and weighs 2.9 carats. Boulder opal is also known as "Type 2 Opal".
Matrix Opal --- (Type 3 Opal)
"Matrix opal" is a term used for rough or finished gemstones in which precious opal is in an intimate mixture with the parent rock instead of the opal being confined to seams and patches as in boulder opal. The specimen on the left is a cabochon cut from matrix opal mined at Andamooka, Australia. The specimen on the right is a bead cut from matrix opal mined in Honduras. This material is often known as "Honduras Black Opal" because of its black base color and pinfire appearance. Matrix opal is also known as "Type 3 Opal".
"Light opal" and "white opal" are terms used for opal material that has a white, yellow or cream body color. This is the most common body color for precious opal. These stones were cut from material mined at Coober Pedy, South Australia. They are calibrated 8 x 6 millimeter cabochons.
"Black opal" is a term used for opal that has a dark body color, often black or dark gray. The term is also used for opal that has a dark blue or dark green body color. The dark body color often makes the fire of black opal more obvious. This contrast of fire color to body color makes black opals very desirable and sold for high prices. This specimen is a solid black opal with a strong blue face-up color play. It was mined at Lightning Ridge, Australia, the "Black Opal Capital of the World". It is 2.46 carats in weight and 9.5 x 12.5 millimeters in size.
"Crystal opal" is a term used for a transparent to translucent opal material that has a play of color within the stone. The stone shown here is a crystal opal with blue to violet play of colors. It is a calibrated 8 x 6 millimeter stone cut from material mined at Lightning Ridge, Australia.
Shown here is a tear-drop shaped stone cut from "Peruvian blue opal" mined in the country of Peru in South America. Although this stone is common opal that does not have a play of color it is nevertheless very desirable because of its beautiful blue body color. This stone is 13 millimeters
by 8 millimeters in size, and weighs 2.3 carats.
Opal also occurs in shades of pink. These pink opal beads were made from common opal mined in Peru. They are about four millimeters across and range in color from nearly white, through carnation pink, thorugh lilac..
"Morado" is the Spanish word for "purple". Some common opal with a purple base color produced in Mexico has been given the name "Morado Opal". The stones at right are nice examples. The stone on the left is a 13x26 millimeter teardrop and the stone on the right is a 19 millimeter round.
"Harlequin opal" is a name given to an opal with patches of fire in the shape of rectangles or diamonds. The specimen at
right is a harlequin opal from the Constellation Mine in Spencer, Idaho. It is 6 millimeters by 4 millimeters in size.
Contra-Luz Color Play
"Contra-Luz" is a name used for a color play that is visible when the light source is behind the stone. This effect only occurs in stones that are transparent or nearly transparent.
"Pinfire opal" is a name used for opal that has pinpoints of fire throughout the stone. The opal on the left is a pinfire opal cut from material mined at Coober Pedy, Australia. The stone on the right is a pinfire opal from the Constellation Mine in Spencer, Idaho. It is 6 millimeters by 4 millimeters in size.
Rarely, opal will have fire that yields an optical effect similar to a cat's eye. In these opals a thin line of fire will be visible from multiple directions and track back and forth across the stone similar to the cat's eye known in other stones. Shown here is a cat's eye opal from the Constellation Mine in Spencer, Idaho. It is four millimeters round.
Opal Names Determined by Geography
Andamooka is one of the early mining districts of South Australia. Commercial production began there in the 1920's. The area is famous for its matrix opal. The stone at right is a cabochon cut from Andamooka matrix and weights about 30 carats.
Coober Pedy Opal
Coober Pedy is a small town in South Australia that was first settled in 1916 when mining for opals began. It was one of the early prolific producing areas and has earned the nickname of "Opal Capital of the World". Coober Pedy is famous for producing white base-color opals and production has continued uninterrupted since 1916. The stone pictured at right are white Coober Pedy opals cut to a calibrated size of 8 x 6 millimeters.
Lightning Ridge Opal
Lightning Ridge is a town in New South Wales, Australia that has become world-famous for its deposits of black opal. More black opals have been produced at Lightning Ridge than at any other location in the world. The specimen on the left is a solid black opal with a strong blue face-up color mined at Lightning Ridge. It is 2.46 carats in weight and 9.5 x 12.5 millimeters in size. The specimen on the right is a solid crystal opal with blue to lavendar play of colors cut as an 8 x 6 cabochon.
Honduras Black Opal
Honduras is well known for producing a black opal with a matrix or pinfire distribution of fire. Most people who know opal will know exactly what you are talking abouut if you use the term "Honduras Black Opal". The specimen at right is a bead cut from Honduras Black.
"Louisiana opal" is a quartzite cemented with precious opal that has been mined in Vernon Parish, Louisiana. On close examination you can clearly see quartz grains with the spaces between them filled with a matrix of clear cement that produces a play of color in incident light. It is a stable material that can be cut into cabochons, spheres and other objects. Some of the material is brown like the 20mm x 20mm cabochon at right but it also occurs in a gray to black color that makes the play of colors easier to see.
Shown here is a tear-drop shaped stone cut from "Peruvian blue opal" mined in the country of Peru in South America. Also some pink opal beads made from common opal mined in Peru. Although these stones are common opal that does not have a play of color they are nevertheless very desirable because of their beautiful blue body color. The blue stone is 13 millimeters
by 8 millimeters in size, ane weighs 2.3 carats. The pink beads are about 4 millimeters in diameter.
Most cut opals are solid stones. The entire stone is cut from a single piece of rough (see top illustration). However, some opal rough has very thin but brilliant fire layers. Some artisans cut the stone down to the thin fire layer and glue it to a base of obsidian, potch or basalt - then cut a finished stone. These two part stones are called "opal doublets" (see center illustration). To protect the soft opal from abrasion and impact a crystal clear top of quartz, spinel or other transparent material is sometimes glued onto the opal. This produces a three part stone, called an "opal triplet" (see in the bottom illustration the clear cap, opal layer and base). The photo below shows two opal triplets, one face up and one upside down.
The two pictures at right are of the same stone. The picture on the left shows the face-up appearance of the stone. The picture on the right is a side view. This stone is an opal doublet that was assembled from a thin layer of precious opal glued to a backing of parent rock. In the side view you can clearly see the "glue line" between the two materials. If this stone was mounted in a setting with a cup bezel it might be impossible to tell if it was a solid opal or a doublet.
The two stones at right are opal triplets produced by sandwiching a thin layer of precious opal between a backing of obsidian and a cover made of clear spinel. The clear top acts like a magnifying lens and enhances the appearance of the thin precious layer. The black obsidian back provides a contrasting background that makes the play of color in the precious layer more obvious. If you look very closely at the inverted stone you will see a tiny link of color that is the edge of a thin slice of precious opal.
Names Used for Opal and Opal Look-Alikes
Because of opal's beauty and desirability people have been producing materials that look like opal for nearly a century. A person with a little experience can easily recognize most of the "look-alikes". "Natural opal" is the name used for genuine opal that has been mined from the Earth. It is genuine opal made by nature and not by humans. The specimen at right is a black opal mined at Lightning Ridge, Australia.
"Synthetic opals" or "lab-created opals" have been made by humans. To be called "synthetic opal" they must be made from a material that has the same chemical composition (hydrated silicon dioxide) as natural opal. Synthetic opals have been made since the 1930's. Some synthetics look very much like genuine opal. However, most are easily recognized by a fire pattern that looks like thin snippets of foil embedded in a glassy matrix or streaks and spots of fire with a geometic shape.
Trade Name Materials
Some synthetic and imitation opals have been sold under trade names that have been at least temporarily popular. A material sold as "Gilson Opal" or "Gilsonite" is a lab-created material with a chemical composition that departs from the hydrous silicon dioxide chemistry of natural opals. "Slocum Stone" or "Slocum Opal" is another variety of opal simulant. During the 1960s and 1970s these and other opal silmulants were popular. These materials can be beautiful with wonderful color. A hint that you are looking at one of these early opal simulants is when you see play of color that is in a patchy to blocky pattern. The stone at right is a Gilson Opal about 20 mm in height.
"Imitation opals" are made from plastic or another glassy substance that is not silicon dioxide. They usually have a pearly "opalescence" rather than a geniune "play of color". Plastic and glassy materials are sometimes called "opalite" when sold in stores.
Most opal will glow or fluoresce weakly under an ultraviolet lamp. However, some speciments exhibit a spectacular fluorescence. This specimen of mossy common opal rough from Virgin Valley, Nevada fluoresces a brilliant green under UV light. The photo on the right was taken under normal light, the photo on the right was taken under a short wave ultraviolet lamp.
Opalite is a name given to an impure variety of common opal that can contain plumes, moss or other inclusions. The name "opalite" can be confused with plastic or glassy materials - imitation opals - that are sold under the same name.
"Water Opal" or Hyalite
Some opal does not exhibt a "play of color", does not have a base color, and does not have a body color like most common opals. But this material is still opal. The tumbled opals at right are examples of this material. It has been called "water opal" and "hyalite"..
Opals on Mars?
In 2008, NASA's Mars Reconnaissance Orbiter has discovered a number of opal deposits on Mars. In the satellite image at right, the ground the surface in the pinkish cream-colored area to the right of the impact crater is covered with hydrated silica rock debris that we would call "opal". Mars researchers have also identified layers of opal exposed in the outcrops of crater walls. Since opal is a hydrated silicate its formation requires water. So, the discovery of opal on Mars is another evidence that water once existed on the planet. Image by NASA.