This is a natural knobby from Lighting Ridge Australia which has been rubbed down by an opal miner to expose the bright flashy blue opal color bar. The opal rub weighed 50cts and it cut a beautiful 9.10cts N1 black opal.
If you prefer to watch how this done, check out the youtube video.
The white Porcelain top has to be rubbed down to expose the colour. These white porcelain tops are rare and most of the time the opal will show bright electric blues.
We have an old Gem master cutting machine that’s around 20 years old and it just keeps going.
With black knobbies it is important to pick which is the up or down side. If both the top and the bottom are black potch this can sometimes be tricky. If you purchase a parcel always cut the lowest grade first and don’t cut the rough with the most potential till you fully understand how the rough faces.
If you have just one piece of rough its best to cut and work a small edge to see how the colour exposes itself.
While rubbing down this knobby on a 400 grit diamond wheel you can see the color darkens inside this white potch. Using a 240 grit wheel is also ok for this stage, just don’t press the opal hard against the wheel or it could crack.
Let the wheel work for you and always have plenty water. Shape the opal on the 800 grit diamond wheel. Shape the stone by hand with a firm grip.
Place the opal on a dop stick so it is easier and more accurate to shape and polish. You can use quick drying super glue and when you are finished cutting you can place the dop in the freezer for few minutes and stone flick off easy.
We have used green jewellers wax and a small methylated Bunsen burner. Quickly heat the back of the stone so it is dry and then heat the wax and slowly drip or smear onto the opal. Than you just need your fingers to rub around edge to make a smooth finish. Many cutters burn their fingers doing this and it does take practice .
Now with dob stick you can control and shape the opal and start getting ready for polishing.
On the last diamond cast wheel do some quick turns. This will ensure and even curved surface on the opal. A good way to see if there are any scratches is to hold the opal up to the light and roll backwards and forwards so the scratches are more visible
Polishing stage starts with a rubber polishing wheel that has 1200 grit and then 1800 grit. Keep plenty of water on the opal and twist opal frequently as they can heat up on these rubber wheels if pushed too hard.
Final stage is to use lapsa polishing powder on the last lap. Pig leather can be used for this or the new Gemshow lapidary polishing disk. use a small amount of polish and be sure to remove all scratches.
Black opal that has been mined at Lightning ridge is cut and polished in a similar style to all other opals. The most important aspect of black opal is to determine which side of opal to cut. This may seem basic information but it is the most common mistake cutters make.
After purchasing parcel of rough opal, never cut the most promising piece rough opal. Always start on small pieces so you understand how the opal will cut. Start by rubbing small corner of piece to determine which side faces the best. Some grey rough will have black centre so also rub on grey pieces before you start on black.
If large pieces crystal opals they will sometimes cut better display colour if cut at angle. It is alot of trial and error and no set rules with rubbing black opals. Except use common sense when cutting black opal. Cut small pieces and understand how the parcel will cut.
Even on the opal fields some opal mines will vary on composition on each layer as opal mined 20 feet down might be different to opal mined 40 ft down.
Another problem new cutters find is that their new polishing wheels are to Sharp and rub away the thin colour bars on black opals. I have seen $800 parcel rough cut with sharp grinding wheels and only cut a $200 stone as the colour was rubbed away.
With new opal cutting equipment it is always best to rub large rock or piece of potch against new grinding wheels. Opal cutting equipment can be home made or purchase commercial opal cutting equipment .
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