When opal buyers visit the Australian opal town of Lightning Ridge, they will be offered buckets of Potch and colour by opal miners or opal runners. Runners are people who pick up opal rough from the miner and show it to buyers around the town. Most of these buckets will look just like rocks to the untrained eye
Potch is what precious opal is formed on. It can be grey, black or even white in colour. The majority of common opal mined is called Potch, so it is basically common opal with no colour. Potch is a hydrated amorphous form of silica and is known as a mineraloid. It is not classed as a mineral.
Potch mined in low depths of around 10 meters are drier than potch for deeper underground. Potch mined down as deep as 30 meters has more loose material or chunks of opal dirt around them so it looks more like rocks with potch veins in them.
It is generally thought that if the opal miner is at shallow depths than the opal is more stable. If buyers see potch that’s full of opal dirt and if the opal dirt is muddy it tells the buyer it was mined at a lower depth. When the miner is digging in opal dirt and he finds traces of potch he will change direction and follow that vein hoping to find precious opal. It may even be in the roof of the tunnel.
When opal miners are testing a new opal field they will drill small holes in a grid formation. They are pleased to find potch as it is good indication that precious opal is close by. If a miner finds potch it is not guarantee that he will find precious opal.
Earthquakes or shifts in level can make many veins of potch come to a dead end or produce no precious opal colors. If you ever get the chance to visit Lightning Ridge you will see lots of people out specking on mullock heaps after the rain. As the rain clears the dirt off the potch and if there is any opal colour exposed it is easy to see. This is called specking similar to fossicking. People look at potch to see if any color veins or colour flash in the piles left over from the opal mining called mullock heaps
Opal color is formed in the internal structures of precious Opal. Precious Opal will diffract light creating a spectrum of colors whereas potch has no diffraction and no opal color. Scientifically precious opal is known as SiO2·nH2O. Opal is made up of tightly packed minute silica spheres only 0.00001 inches’ wide. If these spheres are aligned correctly and just the right size they will produce flashes of opal color. If they are randomly sized and arranged they will be called potch.
Potch at Lightning Ridge is generally a grey color to black. Seam opal fields have more veins of grey potch where as other opal fields have more nobby and rock like formations. Some specimens may look like grey potch on the outside but on the inside display black color.
If the black potch is under the precious opal colour it will result in a rare black opal with body tone of N1 to 4N. But as many opal cutters find out this is rare so the rough black potch might not be anywhere near the precious opal colour resulting in a semi black or grey opal with body tone N5 to N7
When the opal miner has a bucket of rough opal the opal miner will sort through the parcel and take out the king stone or rub down the rough that shows the most potential. As the opal miner knows his mine, he knows which way to cut or how the opal will face. In the opal mining industry now nearly all miners rub down the parcel potch and color to value it and to find the king opals. Lots of rough potch will not be touched and offered as virgin rough. The miner sells the bucket of potch and color after he has graded it.
Many buyers enjoy the chance of buying potch and colour buckets in a chance he will snag a top gem opal. This is one reason why potch and colour is so popular. There is always a chance of finding a gem that the miner has missed but you do a pay premium for the chance to take a gamble.
Many professional buyers buy opal potch and color that has been rubbed down and these are called rubs. Buyers can see the opal colour and estimate what size opal it will cut. This is by far the safest way to buy expensive black opal rough instead of chancing a find of a gem in potch or opal dirt.
We all seem to like to take a gamble and we see so many disappointed results in buying potch and color and losing money or breaking even. Then again some people make a killing by finding a gem opal in potch parcel.
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