Post by gemkoi on Nov 14, 2006 16:07:07 GMT -5
Cutting Koroit Boulder Opal by Shain Mayer (c)2006
First let me state I am know expert on opals, but I have cut a heck of allot of Ironstone opal. So if you see anything in this write up that is incorrect, please feel free to email me the correction.
Now let me talk about Ironstone Opal. Though this cutting is around Koroit Opal, this is an Ironstone Opal. And Ironstone opals are also known as Boulder opals. I have only ever seen Boulder opals come from Australia, but there are a few different types.
Notable Boulder opal is have large pockets of opal in either thin veins(tight) or as thin skin surface or faces of opal. On average there is less than 20% of opal in the ironstone rock. Most boulder opal is more of a display piece and not easy to cut into cabochons because the opal is usually very thin.
Koroit & Yawah Opals are a type locale of Boulder opal in which the ironstone is more of a nodule and the opal is still in vein form, but the veins can be very tight and spread out through the whole nodule. More often the veins form as certain layers were the opal is more concentrated. Both Koroit and Yawah opals produce some of the most intense fire flash out of all Opals in the world. The intensity of the fire is unreal most times. Unlike traditional boulder opal were the opal is rather opaque and dull, but with green or blue coloration, not real flashy like in the nodule opals.
There are more type locale but these three tend to be the more common one can find rough for sale on the open market.
I personally like Koroit the best because I have found more rough than the other two, which also had a high degree of fire per stone. Koroits produce awesome flame like patterns when cut properly and can be some of the most dramatic out of any opal ive ever seen.
-------------------------------------------------------------------------------
Ok, so you got a piece of Koroit eh?
The first thing you need to do is examine the rough very closely and see if you can orientate the fire layer {if there is a fire layer}. The piece here not only do I have a pretty good fire layer visible, there are several spots along three sides that show the same layer peeking through.
Profile--------------------------------Top---------------------------------Bottom
Notice the natural curve in the rough, that’s the nodule form. If you can find a fire layer like I have here in this piece. It will follow that natural curve, so you most follow that curve too in order to cut it properly.
Now, on this piece, though I probably could slab it on the sort end, were fire is already visible. But I suspect the fire wont be deep enough in the layer {at least the bright fire} so Im opting for the rind method.
The rind method basically is started by the removal of the top outer skin layers. Allot of fire agate cutting is done this way. Its very easy.
-------------------------------------------------------------------------------
*Take your piece directly to the coarse grinding wheel on your cabochon machine.
*Look and find the lowest area of the skin to the visible fire layer, cut that skin down till the layer of fire is exposed fully and no bad pits or visible defects from the ironstone.
*Now, as with the piece Im working on, I continue this process around the perimeter of the rough first.
The ideal of working down skin on the edge first to give you an idea were the layer actually lies. If you start cutting the top too much at first. You will find the layer, but you can easily cut to far on top when you begin working the sides and trying to expose the full size of the fire layer.
*Once you basically find the edges of the fire layer. Begin working the center to meet up with the edges. Always take close attention to the vein-lets of fire, are they getting larger, or are they getting smaller. If they fire vein-lets all are getting smaller, stop. Your losing fire. Some may get larger as other get smaller. Take notice when working on the coarse grit. The idea to find a balance on the fire layer is important. Because you still have to sand the piece.
*Now stop coarse grinding before you think the fire is fully exposed. Because now you can take it to the sanding stages of 220 grit. Which will help expose the fire further and sand it at the same time.
*Take the piece as far as you can on 220 grit. That will be the basic shape and dome of your cabochon.
-------------------------------------------------------------------------------
*now trim the back off using a 6inch trim saw.
*refine the shape of the cab, and cut any excess off the back.
In the case of this cabochon, it was thick enough still {and I thought I could expose the back side of the fire layer}, I dome the back of the cabochon. You can easily flat lap the back of your cabochon if you have a flat lap. If you don’t, then take much care in slabbing the stone perform as straight as possible so you can have a flat back and use your grinder to shape the edges and the top a little more.
*The process here is really like any cabochon cutting. Work the piece how you like and always pay close attention to that fire layer. You probably will have better vein-lets in some areas than others. Just be diligent about following the layer as best as you can. Slowing exposing the layer is best. Not all pieces will be as easy as this one either.
-------------------------------------------------------------------------------
Here is the Finished Cabochon from this piece.
A couple extra notes;
*Allot of times the fire is in a small pocket, less than 10mm long and 2mm thick. Which involved lots of creative trimming and or slabbing. If the ironstone is soft {lighter in brown color} it will be easier to cut, but it also wont take a good polish. The darker the ironstone, the better and harder like agate the rough can be. I like the darker stuff just because it takes such a mirror polish. But both lighter, darker, and mixed color grade can exhibit supreme fire flash.
Buying the rough can prove difficult, but it is still readily available through many opal dealers online and off. I do recommend though if you have the money, and can buy the rough in person to do so. As you can inspect the fire layer best this way. Vers buying proven pieces online. However, if you don’t have the in person option, online suites fine. But proven rough that has been trimmed or slab somewhat can be better rough to buy for the first time cutter.
Please email questions or comments about this write. Or post them here to the message board.
First let me state I am know expert on opals, but I have cut a heck of allot of Ironstone opal. So if you see anything in this write up that is incorrect, please feel free to email me the correction.
Now let me talk about Ironstone Opal. Though this cutting is around Koroit Opal, this is an Ironstone Opal. And Ironstone opals are also known as Boulder opals. I have only ever seen Boulder opals come from Australia, but there are a few different types.
Notable Boulder opal is have large pockets of opal in either thin veins(tight) or as thin skin surface or faces of opal. On average there is less than 20% of opal in the ironstone rock. Most boulder opal is more of a display piece and not easy to cut into cabochons because the opal is usually very thin.
Koroit & Yawah Opals are a type locale of Boulder opal in which the ironstone is more of a nodule and the opal is still in vein form, but the veins can be very tight and spread out through the whole nodule. More often the veins form as certain layers were the opal is more concentrated. Both Koroit and Yawah opals produce some of the most intense fire flash out of all Opals in the world. The intensity of the fire is unreal most times. Unlike traditional boulder opal were the opal is rather opaque and dull, but with green or blue coloration, not real flashy like in the nodule opals.
There are more type locale but these three tend to be the more common one can find rough for sale on the open market.
I personally like Koroit the best because I have found more rough than the other two, which also had a high degree of fire per stone. Koroits produce awesome flame like patterns when cut properly and can be some of the most dramatic out of any opal ive ever seen.
-------------------------------------------------------------------------------
Ok, so you got a piece of Koroit eh?
The first thing you need to do is examine the rough very closely and see if you can orientate the fire layer {if there is a fire layer}. The piece here not only do I have a pretty good fire layer visible, there are several spots along three sides that show the same layer peeking through.
Profile--------------------------------Top---------------------------------Bottom
Notice the natural curve in the rough, that’s the nodule form. If you can find a fire layer like I have here in this piece. It will follow that natural curve, so you most follow that curve too in order to cut it properly.
Now, on this piece, though I probably could slab it on the sort end, were fire is already visible. But I suspect the fire wont be deep enough in the layer {at least the bright fire} so Im opting for the rind method.
The rind method basically is started by the removal of the top outer skin layers. Allot of fire agate cutting is done this way. Its very easy.
-------------------------------------------------------------------------------
*Take your piece directly to the coarse grinding wheel on your cabochon machine.
*Look and find the lowest area of the skin to the visible fire layer, cut that skin down till the layer of fire is exposed fully and no bad pits or visible defects from the ironstone.
*Now, as with the piece Im working on, I continue this process around the perimeter of the rough first.
The ideal of working down skin on the edge first to give you an idea were the layer actually lies. If you start cutting the top too much at first. You will find the layer, but you can easily cut to far on top when you begin working the sides and trying to expose the full size of the fire layer.
*Once you basically find the edges of the fire layer. Begin working the center to meet up with the edges. Always take close attention to the vein-lets of fire, are they getting larger, or are they getting smaller. If they fire vein-lets all are getting smaller, stop. Your losing fire. Some may get larger as other get smaller. Take notice when working on the coarse grit. The idea to find a balance on the fire layer is important. Because you still have to sand the piece.
*Now stop coarse grinding before you think the fire is fully exposed. Because now you can take it to the sanding stages of 220 grit. Which will help expose the fire further and sand it at the same time.
*Take the piece as far as you can on 220 grit. That will be the basic shape and dome of your cabochon.
-------------------------------------------------------------------------------
*now trim the back off using a 6inch trim saw.
*refine the shape of the cab, and cut any excess off the back.
In the case of this cabochon, it was thick enough still {and I thought I could expose the back side of the fire layer}, I dome the back of the cabochon. You can easily flat lap the back of your cabochon if you have a flat lap. If you don’t, then take much care in slabbing the stone perform as straight as possible so you can have a flat back and use your grinder to shape the edges and the top a little more.
*The process here is really like any cabochon cutting. Work the piece how you like and always pay close attention to that fire layer. You probably will have better vein-lets in some areas than others. Just be diligent about following the layer as best as you can. Slowing exposing the layer is best. Not all pieces will be as easy as this one either.
-------------------------------------------------------------------------------
Here is the Finished Cabochon from this piece.
A couple extra notes;
*Allot of times the fire is in a small pocket, less than 10mm long and 2mm thick. Which involved lots of creative trimming and or slabbing. If the ironstone is soft {lighter in brown color} it will be easier to cut, but it also wont take a good polish. The darker the ironstone, the better and harder like agate the rough can be. I like the darker stuff just because it takes such a mirror polish. But both lighter, darker, and mixed color grade can exhibit supreme fire flash.
Buying the rough can prove difficult, but it is still readily available through many opal dealers online and off. I do recommend though if you have the money, and can buy the rough in person to do so. As you can inspect the fire layer best this way. Vers buying proven pieces online. However, if you don’t have the in person option, online suites fine. But proven rough that has been trimmed or slab somewhat can be better rough to buy for the first time cutter.
Please email questions or comments about this write. Or post them here to the message board.
... to the New Guys from The ChatBox V.2 Crew