I am just leaving Coober Pedy, source of 70% of the world’s opals and I am feeling at once quite educated and hungry to learn more. Opal forms when silica separates from sand. It simply separates and hardens. It does not need heat, it does not need pressure, it just drips into the empty crevices of the earth and sands and hardens into a rainbow of colors. Mining in the dusty red dirt of the Australian outback, the opal becomes hardly discernible. It is with careful picking and digging that opals are discovered. Given the wealth of opals in this town, one would think they are everywhere. I even found myself walking the dusty streets of this town of 2500 with my head down, waiting to discover my own treasure along the dirt roads. But once I went to the mining fields I realized the truth, that these riches aren't discovered overnight, they take money and time and patience to be discovered. And some miners, poor things, never find any opals at all. Can I get a , “bless their hearts!”
What I heard often in and around Coober Pedy was the discovery of potch. “Oh, that is potch… just potch,” was a common overheard comment. Some would just say, “POTCH!” with a look of exclaimed disgust. Potch is about as close as you can get to opals without being opal. I am reminded of when the undiscerning diner, eating a plate of oysters bites down and discovers a pearl. “A PEARL!” may all their dreams of riches come true! And with zest and zeal the diner hopes they have discovered their fortune only to realize the small pebble which almost broke their teeth is merely some sort of calcium deposit and nothing more. They have not found their riches or even something pretty enough to add to a ring or necklace. There is a reason for this… it is called nacre.
There is something called a non-nacreous pearls. Nacre is the beautiful mother of pearl substance that lines the inside of mollusk shells and forms the beautiful pearl. Some mollusks do not have a gorgeous nacre.. in fact, their shell looks dull and chalky. When these mollusks produce pearls, the pearls are dull and chalky too. Now, to be fair, some look like porcelain. But the shell and the resulting pearl is still thick and lacking the airy essence of nacre rich pearls. The secret is in the crystal structure. Inside of a pearl is a web of interlocking crystals and if these crystals are lined up in a certain structure, the light refracts through these layers and reflects back a beautiful luster. If the structure is jumbled up, it has no beautiful lustrous reflection. No luster usually means no value. How cool that the same is true for opals!
Potch is simply the opal material but without the same crystalline structure. So, without the deep structure, the amazing color, luster and brilliance is not reflected back to the viewer. Potch looks flat and milky. Potch is opal with no color. The light does not reflect back to the viewer in the beautiful rich color opals are known for.
Pretty cool stuff in this desert outback!
P.S. Since there is ALWAYS an exception to the rule, read more on What is a Non-Nacreous Pearl? and When Non-Nacreous Pearls Have Value