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Opal Properties Facts and Photos

precious opal gemstone with play of colour
Contents

1. What is Opal?
2. Opal Meaning and Properties
3. Article Pictures
4. Shop for Common Opal

What is Opal?

Opal is form of silica that contains a significant amount of water.  Under the right geological conditions, as certain rocks break down silica is released into groundwater trapped in cracks and voids.  Over long periods of time as it dissolves the mineral-rich water dries into a gel.

The silica in opal forms uniformly sized closely packed spheres that come together in a regular repeating arrangement.  The spaces between the spheres contains water.  Light diffracts off the spheres as it passes through the stone which produces a rainbow of colours.

Diffraction is the bending of light as it goes around an object.  Refraction is the slowing down of light as it enters an object.
    
Diffraction only happens when the minute silica spheres share a similar shape and size.  They must also be arranged in a neatly arranged pattern.

The quality of the iridescence correctly known as 'play of colour' is dependant on the size of the spheres and the space between them.  The more uniform the shape and size, the brighter and more intense the colour.  The most common colours seen in opal are red, orange, green and blue.  

As an opal stone is gently moved around, depending on the size of the spheres light reflects from different angles.  With smaller spheres blue and violets can be seen whilst larger spheres produce reds and orange.
rough precious opal
Precious opal and common opal are virtually the same stone.  The only difference is one exhibits an optical phenomenon known as 'play of colour', the other does not.  When the shape and size of the silica spheres in the structure of opal differ and the pattern they form is random, there's no diffraction of light.  This stone is known as common opal.

Common opal which is opaque can be found in many different colours but does not exhibit iridescence (play of colour).  The vast majority of the world's opals are common opal.  This stone is not as well known or as popular.  It can be found in many different colours but is best known for being pink or green.  It's called 'common opal' because it's fairly common and can be found in many countries around the world.

When used for gemstones both precious and common opal are polished as cabochons.

Boulder opal forms in thin veins within ironstone boulders.  Ironstone is the host rock.  The main constituents of these fine grained heavy and compact sedimentary rocks are oxides of iron, clay and/or sand.

Freshly broken ironstone is usually grey.  The brown external appearance is due to the oxidation of the stone's surface.  Ironstone can also be found in a red and black banded form.  When polished as a gemstone this material is also known as tiger iron.opal embedded in petrified woodOpal is not crystalline so is classed as a mineraloid or amorphous solid.  To be described as a mineral a naturally occurring solid must have a crystal structure or in other words, be made up of crystals.

Precious opal is fragile so needs to be looked after carefully.  If exposed to heat it can lose moisture and dry out which causes 'crazing'.  This term refers to a network of very fine cracks.

Around 95% of the world's opal comes from the outback deserts of Australia.  The remaining 5% comes from Mexico, Brazil and the U.S states of Idaho and Nevada.

More recently opal has been found in Ethiopia and Mali.

Opal stone was discovered by Australian gold prospectors in 1863.  It is however believed to have been mined much earlier by the Aztecs in South and Central America.

Some reports state opals from Ethiopia were being used as tools as early as 4000 BC.  Factual evidence to support this is fairly vague.

Opal Meaning and Properties

The meaning of the name opal may have come from the Sanskrit word 'upala' meaning 'valuable stone'.  Upala may have come from the Greek 'opallios' which loosely means 'a gem with a kind of play of colour'.

Legends and myths have been associated with rocks and minerals for thousands of years.  A story once told by Australian aborigines claim God came down to earth on a rainbow to bring the message of peace to all mankind.  At the spot where his foot touched the ground the opal came alive and sparkled with the colours of the rainbow.

The Ancient Greeks believed opal could give the wearer the power of foresight.  The Romans revered it as a symbol of hope and purity and believed it could offer protection from disease.

Eastern cultures regarded opal as a symbol of truth whilst ancient Arabs believed the stone came from heaven.  They also believed is acquired the 'play of colour' from flashes of lightning.

During the Middle Ages opal was thought to be beneficial for eyesight.  Some claimed it could render the wearer invisible.  For this reason stones were often carried by thieves.

The French Emperor Napoleon gave his wife Josephine a magnificent opal called 'The Burning of Troy'.  The name came about because of the stone's variation of colour.

Today opal is considered to be a stone of true love.  It enhances positive characteristics for those born under the zodiac sign of cancer.  It's the modern and ayurvedic birthstone for the month of October.

A delicate stone which enhances psychic and mystical visions, it stimulates originality and boosts creativity.

Opal is a stone of reflection that picks up on positive feelings and memories and amplifies them. 

Associated with love, passion and desire, opal can stabilise emotions and gives the confidence to explore intimate feelings. It encourages a positive attitude and brings loyalty, respect and understanding.

Are Opals Bad Luck?

A superstition brought about by the novel Ann of Geierstein written in 1829 by Walter Scott damaged opals' popularity.  The book described it as being an unlucky stone.  Its reputation was gradually restored by public figures including Queen Victoria and French actress Sarah Bernhardt.

Opal is also described as being unlucky because it can crack or become damaged very easily.  If an opal cracks possibly because of being dehydrated, it may be replaced with another stone.  If that's looked after in the same way it too is likely to crack.  This has contributed to the perception that opal is an unlucky stone.

Article Pictures

The opal in the photo at the top of our article is from Andamooka in Southern Australia.  The second photo is courtesy of James St. John.  The last photo is opal in petrified wood.  The opals in photo 1 and 3 are housed in the Natural History Museum Los Angeles.  Both images are courtesy of Stan Celestian.

All of the photos are clickable and redirect to the original non-compressed image.

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