The general rule for opal mining is the ground substructure produces an "opal level" every 10 feet in depth, however, this rule of thumb does not always apply to White Cliffs soil as opal can be found at any level. Miners do generally stick to this rule when mining as it gives a secure band of dirt between the drives (horizontal tunnelling) on different levels. Miners also follow signs that the ground may be good for producing opal. These signs often include slides and level stones although finding these underground does not guarantee opal. White Cliffs opal is hard to find and copious amounts of dirt is moved to find that seam in the wall.
There are different methods of mining:
Windless - Bucket on a rope that is hand filled and hand wound up (uncommon)
Self Tipper - Mechanical structure that brings the bucket up, tips and sends the bucket back down (mostly used)
Blower - Truck like vacuum that sucks the dirt out and usually into a tip truck (Few)
Open Cut - Excavation method (Very Few)
Most mining is still dug by jack hammer and once on an opal seam the hand pick. When a miner is on opal they will dig as much dirt around the seam as possible before removing the seam itself.
TUMBLING AND GRADING
Once the opal has been brought to the surface, miners will then place the rough opal into a tumbler. This is a rotating machine with water that rolls and washes the rough opal. This process aids in the cleaning of the rough opal and rubs of as much loose dirt around the opal as possible. It is then manually processed and graded by the miner. The grading process may include separating the opals into pile based on size and color. Not all opal found is good quality and quite often "potch" is found. Potch is simply - opal without color. Unfortunately potch is not worth a great deal on the open market. Good quality opal from this stage can then be sold to buyers in "the rough".
Once opal has been tumbled and graded it is then easier to see the color bars. Predominately White Cliffs opal is seam opal and occasionally the seam is vertical which means the color bars run across the opal and is hard to face when cutting. Depending on the color of the opal, how thick or thin it runs through the stone, will determine if the cutter will turn that piece of opal into a solid, doublet or triplet. Opal is cut and polished on wheels that are similar to that of a bench grinder, however the wheels have different finer grit and a water dripper to grind opal slowly and not allow the opal to heat up and crack.
SOLID OPAL - This means the opal is one piece usually good thick stone
DOUBLET OPAL - "Double" 2 layers. Opal is glued to a backing of either black microscope glass, boulder opal, or dark potch. The reason for this is either the color bar is too thin making the stone unstable for setting or, the opal is crystal (see through) and the backing allows the color in the opal to shine.
TRIPLET OPAL - "Triple" 3 layers. In a triplet the opal is glued to a backing and a clear cap on top. Again this is usally done as the color bar is very thin, or, the cutter is getting more cut stones out of the one piece. Triplets are usually the cheaper range of opal as the opal content in a triplet can be very thin. However, these are ideal for souvenir jewellery and some rings as they can be more hard wearing with the cap on top.
Once the opal have been cut and polished into a solid, doublet or triplet it can then be set into jewellery. Again there are many different settings and silversmiths that can create just about anything. Most settings that are cast and produced by the hundreds have a "calibrated" size for the opal. This means that during the cutting stage the opal needs to be cut to precise measurements in order to fit the setting. This can sometimes mean the opal has a lot of "waste" and when it is so hard to find in the first place it's not ideal to leave opal in an unusable slurry on the cutting room floor. Most miner/cutters prefer to cut free form, as it has come out of the ground, in order to get the most from their opals. These can then be set by jewellery makers and some claw settings.
Opal pricing depends on many factors. This includes carat weight and size, color (how many different colors), the size of the flash of color, the brightness of the flashes and if the opal has red as this is the rarest of color to find in opal. All this goes into each individual cut piece of opal. Basically the bigger and brighter the opal the more expensive it will be.