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By Reviewed By Thomas Dahlberg

Opal Gemstone Information

Opal Gemstones at GemSelect
Natural Opal Gemstones

Opal Basic Information

Opalescent, the word just rolls off the tongue. A beautiful word with a beautiful meaning - brightness, brilliance, shimmer, sparkle, glitter, sheen, iridescent, the list continues. How did the word come about? There was no other way of describing the dazzling play-of-color displayed by opals, a flaming red leaping from a milky background, a metallic blue dancing in the dark, neon greens waving to and fro, nothing else like it exists.

Very few gemstones have adorned man since the dawn of history but opals alongside diamonds, rubies, sapphires, emeralds, and pearls are one of them. Opals flash the colors of a rainbow as you turn or move the gemstone in a phenomenon known as play-of-color. These precious stones are often up to 20% water and made up of tiny spheres of silica and oxygen formed in cracks in rocks or layers of clay or even inside fossils.

It has also been regarded as the most beautiful and exciting of all gemstones by many. The reason for this is the mysterious play-of-color that is displayed as the stone moves. Add to this the brilliance and variety of the different colors contained within this phenomenon and you have a remarkable gem.

Opal Colors

Opal Color Range
Opal Color Range

Describing the color of an opal is not an easy thing to do. Opals possess all the colors of the rainbow but rarely in a neat pattern like a rainbow, more like an abstract painting using the brightest of colored paints.

Let's start with the basics. Opals are divided (broadly) into two types, precious and common, precious opal displays the 'play-of-color' which makes the opal so famous and desired while common do not. We will pretty much forget about common opal for a while.

Precious opals are further divided into categories and the three main types are White Opals, the most common type of precious opal, Black Opals, which show colors against a dark background, and Fire Opals, which are generally a solid bright yellow, orange or red color.

Now, how do they get their colors? There are two parts to the color of an opal, we have the background color of the gemstone which is created (much like most gemstones) by impurities within the silica structure of the stone.

Then we have the 'play-of-color' which is produced by the silica structure of the gemstone. Opals are made up of millions of tiny spheres of hard silica (imagine a box of ping pong balls but all different sizes) sitting on top of each other. Light hits the spheres and bounces off at different angles and in different directions creating the dazzling light show that give opals their unique look.

Opals come in such an array of colors, including many colors on the same stone that determining the rarest and the most valuable color is a tricky business.

Red is generally thought to be the rarest and most sought out color, so it is the most highly valued. This is because the larger the spheres within the opal, the more red is seen and larger spheres are less common. So the rarity of color tends to follow the rainbow, red and orange the rarest and most expensive and greens and blues being less rare.

Add this rarity of red coloring to the less common black background and you get the most prized opal gemstone. Of course, for valuing, you need to add density of color and patterns to the mix!

Some mines have a committee gathered together each week to value the miner's opals. Several seasoned experts will individually select and price the gems on offer per carat. These prices are then combined to give an average figure and this is the price per carat for the week. The variation from person to person even amongst experienced appraisers is remarkable and shows how difficult it is to value opals.

Opal Patterns

As well as dazzling colors, opals also have a number of patterns which have been identified. Of course no two opals will have the same pattern but some distinguishing arrangements of colors and shapes have been observed and some are more pronounced and admired than others.

The most appreciated pattern is called Harlequin, large sections of color in which each color segment are roughly the same size and shape, like a mosaic or checkerboard. A true harlequin pattern is extremely rare and highly sought after.

Other notable patterns include:

Large sections of color with a random pattern of straight edges.
Random thin ribbons of overlapping color.
Chinese Writing
Thin strips of overlapping color which look like Chinese characters.
Tiny dots or flecks of color.
A flowery looking pattern.
Broad Flash
Large sections of color which flash as the stone turns.

Opal Varieties

As we have mentioned there are two basic types of opal, common and precious. Precious is further divided up.

Similar to describing the color of an opal it can also be confusing when talking about the type or variety of opal. Not including the man made opals there are over 20 well known varieties, even more if you include how they are shaped as a type. We will try to keep it as uncomplicated as possible and concentrate on the more familiar varieties.

The first variety is Black Opal or sometimes Dark Opal, the rarest and most valuable. The term black refers to the general background color, not necessarily completely black but certainly a dark base tone. Against this background a myriad of colors dance in the finest of stones. These precious gems are mostly found in Australia but Ethiopia also produces some examples.

Black Opal Color Range
Black Opal Color

Next we have White or Light Opal the most common variety of precious opal but still beautiful in its own right. White, milky or even transparent these opals may be less bold but the play-of-color can be stunning.

White Opal Color Range
White Opal Color

Another well known variety is the Fire Opal, which as its name would suggest is a yellow, orange or red variety. These bright stones are opals but do not usually have the eye-catching movement of color seen in other types of opal. If the Fire Opal does display play-of-color, usually deep within the stone, they are sometimes called Precious Fire Opal or Jelly Opals and are rare indeed. Mexico is the source of most Fire Opals although Australia does produce some.

Fire Opal Color Range
Fire Opal Color

You may have heard of Boulder Opals which, as with most precious opals, come in a wide range of colors but with this particular type the opal is still attached to the rock on which it formed. The background rock plays its part in the beauty of the opal with dark stone the most popular.

Boulder Opal Color Range
Boulder Opal Color

Matrix Opal is an intriguing gemstone. Flecks of opal are ingrained into little pores or holes found in rocks, usually sandstone or ironstone. The rock is cut and polished so the opal shows as specks and swirls of flashing color.

Other categorized opals include Koroit Opals, Yowah Nuts, Dendritic Opals, Opalized Wood Opals, Hyalite Opals. The list does go on and opals can also be named after their country of origin, Peruvian Opals, Tanzanian Opals and so on or their color, Pink Opal, Blue Opal. There is such a great array of colors, sizes, origins, colors, patterns and natural designs.

We should also mention here that there are synthetic opals made of the same chemical composition in the laboratories and imitation opals just made of plastic. So long as they are clearly identified there is no problem. Then we have composite opals, real opals attached to backing material, such as black potch, ironstone or vitrolite to simulate black opal in what is called a doublet. Or a protective crystal dome on the front plus a backing which is called a triplet opal.

Opal Clarity

Clarity is the degree of transparency and lack of internal blemishes in a gemstone and typically the clearer the stone the more valued it is. There are exceptions of course but this is a general rule. Not surprising, opals differ slightly from these rules - they are a special stone.

Opals tend to be categorized as having transparent, translucent or opaque clarity. If you will excuse the pun, let's look closer.

With the valuable black opals clarity is not an issue in the sense that it needs to have an opaque background on which to show off the colors whilst in a crystal opal we need as much clarity as possible to see through to the play-of-color below. An opal considered cloudy or milky enough to depress the play-of-color of the gemstone is certainly valued less highly.

Boulder Opals are a naturally occurring opal slice or layer with a rock backing and in Matrix Opals the opal material is almost an inclusion in another rock. Therefore clarity and inclusions are not so much of an issue but what makes these particular opals unique and so alluring.

Opals are remarkably moist gemstones, some specimens can be as much as 20% water!

If an opal is left in the sunlight or extremely dry conditions this moisture can evaporate away and leave a fine network of cracks in the stone, something called crazing. Crazing can only be fixed with treatments.

What does an Opal symbolize?

The opal is a passionate stone, associated with desire, lust and seduction, freeing inhibitions and inspiring love. To that end it is a gemstone to help entice an ideal partner, improve your attraction and further a relationship.

Color is an important factor in the spiritual power of a gemstone and as Opals come in every color of the rainbow we know that they have a wide ranging effect on your spiritual and physical well-being.

Chakra meditation

Chakras are the energy centers in your body also referred to as Qi or Prana. There are seven Chakras throughout the body each influencing a particular physical, emotional or mental state and each has an associated color. The seven chakras are as follows, Crown linked with the color purple, Third Eye (indigo), Throat (blue), Heart (green), Solar Plexus (yellow), Sacred (orange) and Root (red).

The color which is most dominant in your opal gemstone will determine which chakra it will have most influence on.

As with its influence on chakras being linked to color so it is with its other spiritual powers, Fire Opals for example awaken passions, get the creative juices flowing, facilitate change, encourage progress and help those who want to leave their mark on the world. Milk or common opals are more calming, soothing and clearing the mind and help in creating a peaceful atmosphere for children and mothers to be.

So we can see each type of the myriad varieties of opal will have different affects. As an overall spiritual stone the opal will bring your emotions to the surface, it promotes self belief and will help you attain your full potential. It stimulates originality and inventiveness and encourages an interest in the arts.

It is the stone of Karma - balance, justice, harmony, teaching us that what we put out we will also receive.

Opal Health Benefits

Opal has long been associated with the health of the eyes, a soothing eye wash of opal elixir will do wonders for sore eyes. Opal can help regulate insulin within the body and detoxify and regenerate the liver. It helps with childbirth and PMS and stimulates the bladder, kidneys and lymph nodes.

It can release muscle tension and pains in the lower back and abdomen. It is reported that it can spark memory and stimulate nerve endings which may be able to treat disorders such as Parkinson's. People suffering from anxiety or panic attacks should perhaps keep an opal gemstone about them and in the same way opals are a meditation stone, calming and centering the mind before meditating.

Opal Prices

Opal Price List

Type Weight range Price range / USD

Black Opal

1ct +

$15 - $40/ct

Opal (Australian)

1ct +

$20 - $1000/ct

Opal (Ethiopian)

1ct +

$20 - $200/ct

Fire Opal

1ct +

$15 - $100/ct

Boulder Opal

1ct +

$5 - $300/ct

Opal Doublet

1ct +

$10 - $100/ct

Cat's Eye Opal

1ct +

$2 - $15/ct

As we have mentioned before appraising the value of opals is not an easy thing. Let's take a deeper look at what goes into the valuation of an opal.

Number one consideration is color. In our section on Opal Color we mentioned Black Opals, Boulder Opals and Fire Opals as the most valuable with a dominant red hue being the most preferred. We can add to this the direction of the color. Opals can change color based on what angle they are being viewed at. Some stones can look fantastically vibrant at any angle so will keep a high price others will look spectacular from one angle but a bit drab from another and hence lose value.

Play-of-color is the most important factor in pricing an opal. Bear in mind a few things when considering purchasing an opal, how much of the stone displays this play-of-color? A small brilliant patch in an otherwise flatly colored stone is less valued than an even distribution of color over the whole surface. Also remember the intensity of the colors, the range of colors and the dominant color!

Then we have pattern, although no two opals will look identical they do have some recognized patterns. Harlequin Pattern is the most expensive, a repeating pattern of rectangles or diamond shapes across the face of the opal, especially in red is ideal. Other notable patterns include bamboo Leaf, Chinese Writing and Floral Pattern.

The base body tone for Black and Boulder Opals is important with the darker the tone the more valuable the stone. Then look at the brightness of the colors, either on top of the base color or just the brightness of the colors in a crystal opal. As you may have guessed the brighter, the better.

Now we come to the shape. Opals come out of the ground in all sorts of weird and wonderful shapes and it is the cutter's job to make the most of this rough gem. The most common cut is a simple oval shape with fronts either low, medium or high domes called a cabochon. They are often cut to calibrated sizes that fit neatly into jewelry pieces. A large Black Opal cut to an oval shape with plenty of play-of-color, especially with a dominant red coloring and a high dome is the most sought after and most valuable.

Because natural opal appears in such strange shapes it can also make some very interesting cut shapes. An imaginative cutter can create some amazing gemstones in what are called free shape opals. Boulder Opals are often cut into striking free shapes making some eye catching rings, pendants or ornaments.

Opals are not often faceted, that is cut with multiple flat surfaces, because it lessens the effects of the play-of-color within the opal but some transparent or translucent stones can be, especially the brightly colored Fire Opals.

Inclusions are the next thing to think about when pricing an opal. As with nearly all gems you want to avoid any gemstones with cracks inside, especially those which reach the surface. The clearer the opal the better the play-of-color can be appreciated, a nice glass-like window to the dazzling colors below is most valued. Some interesting inclusions such as organic matter or unusual minerals can be a plus in certain cases - when making a free shaped centre piece - but as with most gemstone, transparency is highly valued. With such an array of types available opals with different levels of clarity can still reach good prices.

The source of the opal should not affect the price too much, a good stone is a good stone no matter where it comes from. That being said, some mining areas have better reputations than others and certainly Australia and Ethiopia do stand out from the rest. There have been suggestions that opals from certain sources have a higher water content and are therefore more liable to craze or crack but we prefer to judge the opal for what it is not from its place of origin.

The size of an opal can of course affect the price, all things being equal a larger carat stone will always be more expensive than the smaller version. However do not sacrifice color and clarity for the sake of size.

Opal History

Louis Leakey, the famous anthropologist, was exploring the cave of Kenya way back in 1939 looking for evidence of early man. He made a remarkable discovery, precious opal man-made artifacts that dated back 4000 BC, more than 6000 years old! Human beings were crafting precious jewelry from opals thousands of years before the pyramids of Egypt were built.

The name 'Opal' comes to us from the Greek word Opallios and the Roman word Opalus which both come from the Sanskrit word Upala and they all mean 'precious stone'.

Judging by the discovery of worked opals in those Kenyan caves, Ethiopia was probably an early source of these precious gems but the main source for the ancient world was Eastern Europe and Turkey. It remained an exceptionally rare stone until the 1800 when discoveries were made in Mexico and Australia. Nowadays 95% of all precious opals come from Australia.

Where is Opal found?

The Globe

Opal mines can be found in Mexico, Brazil, Honduras and the United States in the Americas and in Ethiopia in Africa, however the vast majority of opals are mined in Australia.

90 to 95% of all the opals in the world come from Australia which began its history of opal mining with the first discovery back in 1849 when Johannes Menge discovered some common opal in South Australia.

It was not until significant strikes of precious opals were made in the late 1800s and early 1900s that things really took off for the Australian mining industry especially when the Europeans recognized their quality and started to buy them.

Now the world's finest opals are found in a number of famous mining sites in Australia. Mintabie, Copper Pedy and Andamooka in South Australia, White Cliffs and Lightening Ridge in New South Wales and Winton and Quilpie in Queensland. Mining is never an easy profession but digging for precious gems in the deserts and mountains of outback Australia is very tough indeed.

Many, many moons ago, about 100 million years to be more precise, there was a massive shallow sea in what is now central Australia. This ocean provided the water needed to create the opals that are now found on what were the shores of this body of water, Boulder Opals to the north, Black Opals in the east and White Opals in the south.

How is Opal formed?

The argument over how opal was formed continues to this day and with varieties such as White, Boulder, Matrix, Fire, and Yowah Nuts and even Fossil Opals seeming so different this is not all surprising. The most followed belief is that opals are formed by water seeping into cracks in various rock formations, evaporating away until all that is left is silica gel-like substance which then hardens over millions of years and turns into one form of opal or another.

Can opals be treated?

In a sense all gemstones are treated in one way or another, when rough rocks are mined from the earth they need to be cut and polished for their beauty to be appreciated. However there are further treatments which may or may not enhance their appearance such as color boosting, heating, painting, dying, resins and waxes, oiling or any application of chemicals. Opinions may differ on the efficacy and ethics of such treatments but no doubt all information must be passed onto any potential buyers.

In general, Opals are untreated gemstones other than the basic cutting and polishing that they receive.

Black Opals are the most expensive so it makes sense that any treatments would be to try to emphasize the darkness of an opal. Smoking an opal will allow smoke particles to enter the pores of the opal and darken its background color. Another way is to cook the opal with some form of acid or sugar to produce carbon which will again darken the gemstone background.

Another form of treatment could be the slicing and dicing that produces the doublets and triplets so liked by jewellers, the opal is not artificially changed as such but it is certainly had more work done than a simple cut and polish.

Heat treatment of gemstones it quite common but heating an opal does not seem to have any real effect on it, neither improving nor spoiling. Opals have a relatively high moisture content so it is thought they must be kept in water or away from sunlight and will explode if heated but this does not appear to be as big an issue as often thought.

What jewelry is Opal suitable for?

Opal Jewelry Ideas
Opal Jewelry

Opals have a Mohs scale rating of between 5.5 and 6.5 compared to say a sapphire which is ranked very highly at 9. This makes opals about the same sort of hardness as glass so this must be considered when thinking about how you are going to use opal.
Find out more about the Mohs scale in our article here!

Engagement rings in a gemstone other than the traditional diamond are becoming very popular these days and opals are a very striking looking gem. If choosing an opal for your ring think about a low dome cabochon to minimise the chance of hitting and cracking it, a protective bezel setting and perhaps a Boulder Opal which is a bit harder wearing than a Black or White Opal.

In less often worn jewelry, Opal makes a great choice because of its dazzling beauty and variety. Pendants, earrings and necklaces also have less chance of being 'knocked about' so make a good pick for your gem.

If you are setting an opal gem in a metal, the choice of metal will come down to a matter of taste but the variety of colors mean there will always be an alloy to match.

The large Black, Boulder and Matrix Opals can be perfect for statement making handmade pieces that will really stand out from the crowd. They can be found in the usual oval shapes but also lend themselves to some outlandish shapes and forms with textures and depths to contemplate, too.

Did you know? Interesting facts about Opal

  • Opals are the birthstones for October.

    With the variety of styles and colors there is bound to be one that would suit a loved one born in this month as a gift. Or why not treat yourself if you were born in October.

  • Coober Pedy is one of the greatest opal mining sites in the world but how did it get its name?

    Not from Mr Pedy as you would expect but from the aborigine phrase 'kupa piti' meaning 'white man in a hole!' This could refer to either the early prospectors digging for opals or the fact that the new settlers in this area often dug holes to climb into to escape the intense heat. The town was named Coober Pedy in the 1920s.

  • Half of the population of Coober Pedy still live underground to keep cool during the summer months to this day.
  • Another famous Australian mining town, Lightning Ridge also got its name in a strange fashion.

    Now it is the source of most of the world's Black Opals but back in the 1870s a lightning strike supposedly killed a farmer, his dog and 200 sheep and that's how it got the name!

  • Opal is the 14th anniversary wedding stone. A perfect gift for your husband or wife.
  • Opals have been found on Mars! Conditions on this barren planet have been compared to modern day Australian deserts so we should not be surprised.
  • The opal gemstone industry was nearly ruined by a book.

    Sir Walter Scott's novel, 'Anne of Geuerstein', written in 1829, contains an episode where holy water splashes on a young girl's opal gemstone and within days she is dead! Sir Walter's books were so popular in those days that publicity raged about the misfortune of the heroine and blamed it on the bad luck of opals. Sales plunged for decades.

  • Queen Victoria helped the recovery of the trade by presenting her family with gifts of opal jewelry and wearing it often herself.
  • What is the most valuable color combination?

    The rarest and most valuable of all the opal color combinations is red and blue together, they come from opposite ends of the color spectrum so finding an opal with such a spherical construction would be almost impossible.

  • Sometimes opals can be double-sided with different colors on each side, a very unique gemstone.
  • Australian town of Coober Pedy looks like something straight out of a movie—probably because it is. Mad Max Beyond the Thunderdome was filmed there, starring Mel Gibson and Tina Turner in 1985.
  • How rare are Opals?

    Opal material is quite rare on this planet and about 95% of all opal mined from the opal fields are common or potch, a basically single colored rock of no real value. The remaining 5% has some color, but about 95% of that is of mediocre grade. Therefore, only approximately 0.25% has any real value at all.

How to care for Opals

At 5.5 to 6 in hardness opals are relatively soft for a gem but durable enough to survive the odd bump or knock on a hard surface however you should always take care to avoid such abuse. That is why it is always recommended to remove jewelry before physical activities, especially household cleaning, sports or gardening.

Opals come in two types of cut and polished stones, solid stones and doublets or triplets. Taking care of them differs a little. Solid opals cannot be damaged by water (assuming you are not blasting them with some sort of hydro-cutter) but can be harmed by very dry conditions or rapid changes in temperature. Deposit boxes in banks are notoriously arid areas so a plastic bag with a damp cloth will be a good safeguard from damage. Try not to leave opals exposed to long periods in sunlight or a dehumidified room.

Doublets and triplets are a slightly different matter as they involve glue and backing material so a long soak in water is not a good idea as it can lift the backing or fog up the top crystal layer.

When storing your opal care should be taken to keep them away from any harder materials which may scratch, chip or break them. Keep them in individual cloth bags or boxes for extra protection.

Careful: ultrasonic cleaners can not only cause and exacerbate fractures in gemstones, they can loosen jewelry and cause gems to fall out of their settings. Steam cleaners should not be used to clean amethyst because it should not be subjected to high temperatures. Clean your opals using a soft cloth and only mild soap or detergent if needed.

How do you know if you have a real Opal

Is this gem real?

Obviously a certified gemstone from a reputable dealer is the best approach but this is not always possible when you are out searching for a great gemstone or a bargain!

As with all precious gemstone, there is the temptation for unscrupulous dealers to try to pass off synthetic stones or fake stones as the real thing. Opals are no different although the tricky chemical structure of an opal makes this quite a challenge.

Let's look at doublets and triplets. These are not fakes in anyway unless someone is trying to sell it as a solid stone. First check the body tone, if it is white or transparent then it is unlikely to be a doublet or triplet as they have a black backing to create a dark background.

Look at the stone from the side (if the gemstone is tucked away inside a setting ask for it to be removed) if you see two or three layers you likely have a doublet or triplet.

Examine the back of the stone. A hard plastic coating suggests a triplet and is easy to spot but a natural piece of potch or rock makes a more difficult prospect. Potch is a piece of black or colorless opal material that is not considered of any real worth. Again check from the side, if the join between the opal and the backing is flat and even then it was probably glued together, if it is bumpy and convoluted then it is probably natural.

From the top, the opal will look a bit glassy if it is a triplet and if you can see through the top of an opal from the side that is another indicator. Again, doublets and triplets can be fine pieces so long as they are revealed as such.

Synthetic opals were created in the laboratory in 1974 with an iridescent coloring and the same year saw the introduction of glass opals with metallic foil providing the color. Opalite is yet another imitation, this time made of plastic. There are also ways of 'growing' opals using chemical reactions and these can be indistinguishable from the real thing but are not seen as a threat to the gem industry. The man-made versions vary in quality but the coloring can give away their secrets, the patterns are often too perfect, there will be no gaps in the patterning like real gems and there will probably be an orderly pattern repeated across the face of the stone.

Price as a guide. Opals are a rare semi-precious gemstone so the price should reflect this. If you see a prime example at a price too good to be true, it probably isn't true, so avoid buying such 'bargains'.

This is not a complete guide on how to spot a fake gemstone but I hope it helps.

At GemSelect, we currently offer brief identification reports from your choice of two well-respected independent gemological laboratories, The Asian Institute of Gemological Sciences (AIGS) and Burapha Gemological Laboratory (BGL Lab).

What is so special about an Opal?

If the diverse colors found within a single top quality opal gemstone were not enough to make this gem unique then surely its crystalline structure does. Singularly, the opal is amorphous which means it does not have a defined crystal arrangement so can come in all sorts of strange forms. Its tiny spheres trick the human eye into seeing a bright spectrum of colors by not allowing light to pass through, instead bouncing back every color of the rainbow. Finally, opals were created by rain (so the latest theory goes, anyway), the rain trickled down the rock face and seeped into the cracks then dried up leaving a silica gel which over the millennia hardens into opals. Now that is special!

Can Opal change color?

This may seem a strange question when discussing opals with their incredible play-of-color displays but this is slightly different, some gemstones show a distinct or dramatic change in color under different light sources. Look at a garnet under electric or artificial light and it could look red, take it outside into the sunlight and all of a sudden it is green! This remarkable effect only occurs in a few gemstones, Alexandrite, color-change Garnet and color-change Sapphires being the most well known but does NOT occur in opals.

Opals change color as they are viewed from different angles but not really under different light sources although if you look at some opals under fluorescent the play-of-color can be diminished because the fluorescent lights do not possess the full spectrum of color, so the display by the opal will be missing some hues.

How can you tell a good quality Opal

The types of opal available is almost unbelievable in its variety and everyone's taste and preferences are different so how can we say what makes a good opal? We have discussed how to tell a real opal and which color is most valuable and where is the best source. Let's assume the gemstone comes from a reputable source (a very important factor in choosing a good quality opal) and it is not fake, can we sum up what makes a good stone? First do you like the look of it? Does is feel comfortable in your hands? Can you sense its power and longevity? You have found the stone for you.

Let's make a checklist so you know what you are getting for your well earned money.

Determine the type of opal, Black, White, Crystal, examine the play-of-color and its intensity, coverage, the main color and pattern, then look at the transparency, not too cloudy, no cracks and not too many inclusions. Finally the cut, is it symmetrical enough for what you need? The right thickness, length and breadth? Has the polishing given the stone the sheen you prefer?

Remember you are the one who has to live or work with the stone so the final choice is always yours.

Opal - Gemological Properties

Chemical Formula:

SiO2_nH2O - Hydrous silicon dioxide

Crystal Structure:

Amorphous; kidney or grape-shaped aggregates


All colors, partial play of color


5.5 to 6.5 on the Mohs scale

Refractive Index:

1.37 to 1.52


1.98 to 2.50




Transparent to opaque

Double Refraction or Birefringence:



Waxy to resinous


White opal: white, bluish, brownish, greenish; fire opal: greenish to brown

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