This is a great guide for cutting Black Opal. This article contains thorough information from an expert opal cutter, who has a strong passion for black opals and has been in the trade for many years. He provides information on all the necessary equipment needed to cut black opal. See an inside perspective of cutting, shaping and finally, polishing. There is a special section on tips and tricks for cutting a black opal the best possible way. Black opal is considered to be the ‘Queen of Gems’. Historically, it is highly sought after by kings, queens, emperors, maharajahs and sultans. Its’ unique beauty is something to be marvelled. This guide is perfect for all levels. If you are a beginner black opal cutter, it has pictures for every step of the way, however If you have been cutting for a while, it can still be very helpful to refresh your opal cutting knowledge. We hope you enjoy this guide.
This is a natural nobby from Lighting Ridge Australia. It has been rubbed down by an opal miner to expose the bright flashy blue opal color bar. The opal rub originally weighed 50cts and it cut to a beautiful 9.10 carat N1 black opal.
The white porcelain top has to be rubbed down to expose the colour. 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 is around 20 years old and it just keeps going.
With black nobbies it is important to find which side is up or down. Sometimes it can be very tricky if both the sides are black potch. If you purchase a parcel, always cut the lowest grade first and avoid cutting the rough with the most potential until you fully understand how the rough faces. If you only have one piece of rough, it is best to cut and work a small edge to see how the colour exposes itself.
While rubbing down this nobby on a 400 grit diamond wheel you can see the color darkening inside this white potch. Using a 240 grit wheel is also fine for this stage. Just don’t press the opal too hard against the wheel or it could crack. Let the wheel work for you and always have plenty of 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 a quick drying super glue to stick the opal on the dop stick and when you are finished cutting you can place the dop in the freezer for few minutes and stone will flick off easily.
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 the dob stick you can control, shape the opal and start getting it ready for polishing.
On the last diamond cast wheel it is recommended to do some quick turns. This will ensure an 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. The 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.
The final stage is to use a lapsa polishing powder. 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.
Cutting challenge - this firey black opal rub had black potch in the face which was slowly rubbed off. I had expected to cut a pendant stone but the opal told me where to cut. The sand spot was deeper than I thought and at the edge of the stone the black potch started to appear and would grow bigger if i continued polishing. Instead of a pendant stone i got a stunning black opal ring stone with a lot of fire. It is such a pleasure to cut stones like this!!
VIEW THREE ROUGH BLACK OPALS BEING CUT OPEN EXPOSING STUNNING OPALS
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 the opal to cut. This may seem basic information but it is the most common mistake cutters make.
After purchasing a parcel of rough opal, never cut the most promising piece of rough opal. Always start on small pieces so you understand how the opal will cut. Start by rubbing a small corner of the piece to determine which side faces the best. Some grey rough will have a black centre so also rub on the grey pieces before you start on the black.
If you have large pieces of crystal opals they will sometimes cut better display colour if cut at an angle. There 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 come across is that their new polishing wheels are too sharp and rub away the thin colour bars on black opals. I have seen an $800 parcel rough cut with sharp grinding wheels and only end up with a $200 stone as the colour was rubbed away.
With new opal cutting equipment it is always best to rub a large rock or a piece of potch against new grinding wheels. Opal cutting equipment can be home made or you can purchase commercial opal cutting equipment.