Opal

The name Opal is derived from an Indian (Sanskrit) word ‘Upala’ meaning precious stone. The play of color in opal is shown only when light is reflected from the stone. The play of color is usually absent in transmitted light.  Opal contain 3 to 30 percent of water. Some of the stones lose water at ordinary temperatures and may develop cracks and diminish play of color. A number of varieties are recognized in the market so it is divided into three sub groups - Precious opal, Fire opal, Common opal.

 

 

  1. Precious Opal

    • White Opal – A precious Opal of white or light basic color with play  of colors.
    • Black Opal – A precious Opal with dark grey, dark blue, dark green and grey-black basic color and play-of-colors. Deep black opal is an exception and rarer.
    • Opal Matrix – Banded growth or leafed inclusion of precious opal in the matrix rock.
    • Boulder Opal -  Precious Opal with dark base surface, play-of-color, and high density occurs as pebble rock, where Opal fills hollows.
    • Harlequin Opal-  Transparent to translucent precious Opal with effective mosaic like color patterns counted among the most precious opals.
    • Jelly Opal – Bluish-gray precious opal with little play-of-color.
    • Crystal Opal – Transparent with strong play-of-color on colourless and luster of the surface is vitreous.
  2. Fire Opal-Fire Opal is orange or reddish-orange in color. It often shows no play-of-color .It is usually milky and turbid. The best qualities are clear and transparent which makes them suitable for being faceted.
  3. Common Opal- Common opal is opaque, semi-transparent to translucent and it shows no play-of-color. A number of varieties of trade names are used.
  4. Agate Opal – Agate with light and dark opal layers.
  5. Angel skin Opal – An opaque, Whitish to pink coloured silicate mineral.
  6. Wood Opal- Honey-yellow, translucent Opal.
  7. Hyalite – Colourless, water clear Opal with strong sheen. It is also called glass opal or water stone.
  8. Hydrophane – A milk Opal, which has turned turbid due to the loss of water. Through absorption of water, it can become translucent again and have play-of-color.
  9. Porcelain Opal-White, Opaque milky Opal.
  10. Moss Opal – Kilk Opal with dendrites.
  11. Girasol Opal – Almost colorless, transparent opal with a bluish opalescence.
  12. Prase Opal – Apple Green Opal.
  13. Wax Opal- Yellow-brown Opal with wax like luster.

DEPOSITS

At the end of the 19th century, the Andesite lavas in the east of Slovakia supplied the best quality Opals. Then the Australian deposits were discovered. Famous deposits in new south whales are at Lightning Ridge and White Cliffs; in South Australia at coober pedy and Andamooka. Numerous deposits are also found in Queensland. Further deposits are found in Brazil, Guatemala, Honduras, Indonesia, Japan, Mexico, Russia, Nevada and Idaho. Deposits of fire Opal are found in Mexico as well as Brazil and Western Australia.

PROPERTIES

Species Opal
Variety Opal
Chemical Composition SiO2 + nH2O (Hydrous silicon di-oxide)
Crystal System Amorphous, hardened jelly.
Color Exhibits play of color
Cause of color   Diffraction of light. Green - nickel.
Hardness 5.5 to 6
Specific Gravity 1.98 to 2.20
Pleochroism None
R.I. Range 1.44 to 1.46
Optic Sign None
Birefringence None

INCLUSION

Uniform spheres of amorphous silicon.

FAMOUS OPAL

 
The Olympic Australis : The largest gem Opal in the world in the Olympic Australis, it is about 11 inch long 4.15 inch tall and 4.5 inch wide and the weight is about 17000 Carat. The Olympic Australis was found 1956 at a mine in South Australia’s Coober Pedy. That year Olympic games were being held in Australia. So it was named after the Olympic games.

 
The Aurora Australis: The Aurora Australis is a black opal, and the most valuable in the world. The gem stone was found in Lightening Ridge, in New South Whales. The Opal is a black background, interspersed with green, blue and red colors. The gem was named Aurora Australis after the southern lights.

 
Pride Of Australia / Red Emperor:The pride of Australia or the Red Emperor was discovered in 1915. It is called the Pride of Australia because its shape resembles of that continent. The Opal has blue and black veins, interspersed with red streaks. Unfortunately, the pride of Australia was stolen in 1961 from the Forest Lawn memorial Museum, USA, where it was displayed.

 
The Queens Opal: The Queens Opal is also known as the Andamooka Opal. In 1954 when the South Australian government made it known that it was looking for the best opal ever mined in the Opal fields of of South Australia to present to the queen Elizabeth II. The queen was stated to visit Australia for the first time. Altmann & Cherny had submitted a rough opal from a mine in Andamooka. The stone was chosen by the government and it was cut and polished at Altmann & Cherny. Its weight after it was cut was 203 Carat and shone with colors like blue, green and red. It was set into a palladium necklace.

 
The Halley’s Comet Opal: The Halley’s Cornet opal is an uncut rare black opal. It was mined in 1886 and it was named after miner Halley’s cornet.

SIMULANTS

Opal simulants have come in a variety of forms. They can be made by forming colorless non-siliceous particles into microspheres of the correct size and allowing them to settle , when diffraction colors may be obtained. A simulant using a synthetic resin  for this purpose has been made in Japan. It has the appearance of a light colored coober pedy Opal, and is very deceptive. Metallic flakes, gas bubbles and schlieren are often found in the synthetic Opal. Gilson Opal, Glass, Plastic, and opalescent quartz are also simulant.

 

 

Raw Stone
After Cutting