SOMETHING ABOUT OPALS

  Australia has around 90% of the World's Opal with the majority coming from Coober Pedy. (0% of Opal found is common Opal with only the remaining 10% showing colour.
he Mining of Opal begins with a mining permit with which a Miner can peg a claim 50m x 50m or 50m x 100m from where he will search. Today, mine prospecting shafts are made by using a Calwell type drill to excavate holes about 1 metre in diameter. The drills can reach a depth of 28 to 30 metres. the Opal fields are pitted with thousands of abandoned Calwell shafts. The earlier form of mining was to sink a shaft with a pick and shovel then driving a tunnel along the level in the same way. Traces of Opal were then removed by hand with a pick or screwdriver. Waste material or mullock was originally lifted to the surface from shafts by hand windless, then later by power winches called Yorke Hoists or bucket tippers. Today, truck mounted blowers, which operated like vacuum cleaners are more commonly used to bring mullock to the surface. Since the 1970's more modern methods of mining have been introduced with tunnelling machines and small underground front end loaders or bloggers now in use. All Opal fields have restricted entry due to the thousands of open shafts that exist. permission must be obtained. The is a strict rule around here "NEVER WALK BACKWARDS".
  Another type of mining called "Noodling" is carried out by many locals for a living and is also popular with tourists. the process is that of simply searching through heaps of Mullock for pieces of opal that miners may have missed. Permission must be obtained from the claim owner to fossick on his claim. Noodling machines in which Mullock is passed through a darkroom on a conveyour belt beneath ultra violet lights are often used to detect Opal.
  what about Opal itself? It is a form of Silica, similar to Quartz but with water in it's molecular structure. Opal occurs either as Precious Opal or Common Opal known as Potch. precious Opal in general has 6 - 10 % water content. The colour is caused by a regular array of Silica spheres diffracting white light into the various colours of the spectrum. The and spacing of the spheres controls the colour range. Opal colours also depend on the angle of light incidence and they can change or disappear when the gem is rotated. In Potch, the silica spheres are irregular in shape and molecular structure and are unable to diffract light and consequently there is no colour.
  The main factors influencing the price of an Opal are:

Base Colour: Black Opal is more valuable than Crystal Opal which is more valuable than White or Milky Opal.

Dominant colour:  Red-fire Opal is more valuable than a predominately Green Opal which is more valuable than an Opal which is only Blue.

Colour Pattern: Harlequin Opal, where the colour is in defined patches is usually more valuable than Pin Fire Opal where the colour is in small specks.

There is a big difference in the value of Cut and Uncut Opal as one would expect. There are three main ways that Opals are cut and polished:

Solid (Cabochon): Opals that naturally occur in one piece having been cut and polished from rough Opal.

Doublet: When there is only a thin veneer of Opal, it's co;lour may be enhanced by cementing either black or grey silica, glass or a thin slice of Potch to the back of the coloured Opal.

Triplet: To protect the Opal from damage, a slice of Quartz Crystal is used to cap the thin Opal veneer on a Doublet, thus producing a three tier gem. this can produce a brilliant display of colours and is a cheaper method of presentation that can enhance the appearance of the Gem.

Finally, The "Olympic" from Coober Pedy is said to be the World's largest piece of uncut Precious opal and weighs 3.5 kgs. It is so named because it was found in 1956, the year of the Olympic games held in Melbourne.