Most people have seen opal nobbies if they have bought Lightning Ridge opal. one type of rough opal that has given confusion about how they can cut are WITCHES HATS or CAPS are what they are called. They are an opal nobby that has the appearance of a Chinese straw hat (like the photos above). The opal color inside the hat can be amazing color depending on which opal field in Lighting Ridge it comes from.
One opal fields in Lightning Ridge has the best caps in the Cooceran opal field is called T-bone opal mining field. T-bone opal has been renowned for producing some of the best red on black opal in the world.
The opal caps that come from this opal field can produce nothing cutable anything at all as well, but can show amazing color from the outside. Even sometimes you will have sand right in the center of the opal nobby, which cannot be seen from the outside. This is the challenge with this type of black opal and rubbing your way into the rough opal can be challenging.
The best way to get into a witches hat nobby is to make your way around the colorbar with the cutting wheel, so you can see if the color bar is straight. If the bar is not straight then you will probably find the the stone will be smaller than originally first thought. By using a laser pointer you should be able to see partly into the nobby. Or you can use a bright light to help you look into the color bar. Make sure the colorbar has no more impurities before you face the opal nobby, so you can make an educated guess into where the stone will sit.
Matrix is another type of unhealthy opal that can form in a witches hat nobby and by leaving any Matrix on a solid opal can be disastrous. The opal can crack once it has been cut and polished. Matrix is an unhealthy chalky looking opal that is either grey or white. It has a density that can put pressure on the good quality opal and can cause it to craze. But not in all cases. So taking away can assist in the life of an opal. Having said that I have had opal that has had color and matrix on it and has never cracked. I'm just saying that there is a bigger chance.
My experience tells me that I have to make sure that I orientate the opal nobby so that the backing of the opal solid is potch not matrix. So sometimes you will have to cut the opal the opposite way to what looks obvious.
Upside down witches hats are also common. Where the matrix sits on the opposite side of the color-bar than where the potch usually sits. So knowing which side is the matrix can be very important.
If you have any comments on what I have said or not understood me properly? please Email me to help you understand.
