Opal is formed in dry areas such as Australia’s semi-desert “outback.” Seasonal rainwater drenches the dry regions and showers soak deep into ancient underground rock, carrying dissolved silica (a compound of silicon and oxygen) downward, forming deposits in cracks between rocks. During dry seasons much of the water evaporates, leaving the silica deposits underground in the cracks and fissures of sedimentary rock.
Depending on the conditions in which it formed, opal may be many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these, black opals are considered the most rare while white and greens are the most common.
As with their color opals vary in optical density from opaque to semitransparent, and there are two broad classes of opal: precious and common. Precious opal displays play-of-color while common opal does not.
Play-of-color occurs in precious opal is due to the way in which silica is arranged on a sub-microscopic level. Opal is made up of sub-microscopic silica spheres that arranged in a grid-like pattern (think of it like a box of ping pong balls). This arrangement causes light to bend (diffract). As the light bends it breaks apart into spectral colors (all the colors of the rainbow) producing an incredible display of color. The color that can be seen varies with sphere size. According to GIA "spheres that are approximately 0.1 micron (one ten-millionth of a meter) in diameter produce violet. Spheres about 0.2 microns in size produce red. Sizes in between produce the remaining rainbow colors".
What are Opalized Fossils?
Opal usually forms within fissures in rock layers, but opal can also form in other materials. If a cavity has formed because a bone, shell or pine-cone was buried in the sand or clay that later became the rock, opal may form in these cavities.
One of the richest sources of opalized fossils is Lightning Ridge in northwestern New South Wales. These fascinating fossils transport us back in time approximately 110 million years! The fossils of dinosaurs, marine reptiles, fish, early mammals, mollusks, and plants give us a window into our planet's history.
There are two kinds of opalized fossils:
Type 1. In this type the internal details are not preserved. The water and silica solution fills an empty space left by a shell, bone, etc. that has disintegrated away and then hardens to form an opalized cast of the original object. The opal inside doesn’t record any of the creature’s internal structure. Most opalized shell fossils are this type of ‘jelly mold’ fossils.
Type 2. In this type internal details preserved: If the buried organic material hasn’t rotted away and a silica solution soaks into it, when the silica hardens it may form an opal replica of the internal structure of the object. This happens most often with with wood or bone.
The Virgin Rainbow
The Virgin Rainbow, it was discovered in the opal fields of Coober Pedy by opal miner John Dunstan in 2003.
It's actually an opalized fossil, from an extinct cephalopod called belemnitida that existed during the Mesozoic era. During that time, much of South Australia was under a vast sea that was filled with prehistoric aquatic life. When these creatures died they sank to the bottom of the sea and were buried by sediment.
Over the millennia the sea eventually dried up and the land turned into a desert. The acidity levels in the shallow top layer of the sandstone increased causing the release of silica from weathering sandstone. Groundwater then carried it down to the layer of clay beneath, where bones and pockets left by disintegrated bones lay buried.
Further weathering lowered the acidity levels allowing the silica gel to harden into opals in the pockets and impressions left by decayed animal material to create a replica of the internal structure (see a description of opalized fossil types above).
The famous Australian opal fields of Coober Pedy are located in this prehistoric sea region. No other environment in the world is known to have undergone this same process, which very well may be why over 90 percent of the world's opals come from South Australia.
Opalized fossils in this region are not uncommon. The South Australian Museum is home to a spectacular (almost complete) opalised skeleton of a six-metre (20-foot) plesiosaur known as the Addyman Plesiosaur, although you have to look closely to see its opalescent sheen. This is not the case with the Virgin Rainbow.
"You'll never see another piece like that one, it's so special. That opal actually glows in the dark -- the darker the light, the more colour comes out of it, it's unbelievable," said Dunstan.
"I've done a lot of cutting and polishing [of opals], I've been doing it for 50 years, but when you compare it to the other pieces that claim to be the best ever, this one just killed it."
The Virgin Rainbow is considered to be the finest opal ever unearthed and is worth more than $1 million. (In case you were wondering the Virgin Rainbow isn't even the most valuable opal in the world! A massive, 3.45-kilogram (7.6-pound) stone named the Olympic Australis opal, excavated in the Coober Pedy in 1956 gets that honor. In 2005, the Olympic Australis was valued at $2.5 million.)
Both the Addyman Plesiosaur and the Virgin Rainbow are currently on display among a dazzling collection of opals at the South Australia Museum in an exhibition, called "Opals". The exhibition began in September 25, 2015 and will run until February 14, 2016.
"From jewellery to fossils to specimens embedded in rock, visitors will be treated to a spectacle of unmatched colour and beauty. This is an exhibition literally millions of years in the making because these opals were formed back when dinosaurs walked the earth and central Australia was an inland sea," - Museum Director Brian Oldman.
Aboriginal Origins of Opal
We Came From the Land
This aboriginal story teaches how areas around the Flinders Ranges were created and the origins of opal. This is a Wirangu story from near Ceduna on the west coast of South Australia as told by M. Miller and W.J. Miller.
A long long time ago, a huge meteorite hurtled towards the earth from the northward sky, and smashed into the ground near Eucla. Because it was so big, a dent appeared in the crust of the earth and the meteorite bounced high into the air and out into the Great Australian Bight where it landed with an enormous sizzling splash. It was hot from its trip through space so it gave off a great deal of steam and gas as it sank through the waves. But this was no ordinary meteorite. It fact, it was the spirit Tjugud.
In the deep water near by, the spirit woman Tjuguda lay asleep. All the noise around her woke her up and she was very angry. She bellowed and the elements roared with her. The wind blew, the rain pelted from the sky and the dust swirled.
From the joining of the two spirits, the Tjugud and Tjuguda, a man was born, but he was no ordinary man, he was of enormous proportions. He rose from the deep water of the Bight to swim through the maze of limestone caves which run through the earth and into the sea. Then, he emerged from the ground through the cave of the Nullabor.
This was the birth of the Wirangu man, a coastal dweller. Wirangu walked towards the east, taking huge steps in keeping with the stature of the man. Each time he stepped, the ground shook and a dent appeared in the earth. These would later fill with water and are the rock holes which can still be seen today. You can clearly trace the journey of this man.
When he reached Coober Pedy, he was very hungry so he found some food and then lit a fire. The fire he built was so fierce it burned with an enormous amount of heat. A lot of water from the body of the man dropped into the ground and was captured by the stones which held a lot of water anyway. The beautiful colours from the raging fire went down into these stones, changing the water into a magnificent display of color. This is the colour of the opal and can be found in the stones still.
(Education Department of South Australia 1992: 32-33)
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