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Shaping Boulder Opal

Shaping Boulder Opal

Posted by Therese Brown on 5th Aug 2021

Boulder opal is a unique form or opal that is hosted by rock – ironstone or sandstone. The opal runs through the rock in veins. These veins may be thick, thin, straight, or crooked. The object of cutting a boulder opal gemstone is to expose the ‘face’ of the opal vein. We do this by ‘blocking out’ the outer edges of the vein and exposing the opal face through removing the rock from one side then polishing. The other side becomes the back of the gemstone.

Sawing

Once the rough boulder rough has been sorted and potential cutters identified they are placed in a plastic container beside the saw. My preferred saw runs a 10-inch sintered blade.

The sintered blade is characterized by having an alloy rim containing fine industrial diamonds which is approximately 10 mm (1/2 inch) deep (see opposite image). The saw has a water feed at the top of the blade and I free hand cut at the bottom of the saw blade. My saw is not a bench top saw, but more like a lapidary styled brick saw. This allows more flexibility in addressing the blade, however as the rock being cut is held in the fingers with the hands firmly planted on the base of the saw, it requires a focus to keep the saw-cut square with the blade. I will cut rocks up to 3 - 4 fists in size this way.

10-inch sintered saw. Drop Saw. Note must have a water feed.

A bench saw can be used to break the boulders down if you don’t have a drop saw.

I use a notched blade on my bench saw as a notched blade is less flexible and more accurate for slicing and blocking out.

Notched blade. Always use water when breaking boulder opal down.

Grinding

Once the veins have been blocked out, we move to the grinder. We use diamond grinding wheels with grits that range from 180 to 1200.

The heavier grits are good for removing the rock from the face of the opal vein however be careful not to touch the actual opal with a grit heavier than 320.

The opals veins can be very thin, so it is best to go slow and go from 320 to 600 to 1200 on the grinder.

Always use water when cutting opal. Heat can cause cracking.

Once you’ve ground as much rock off the opal face as you can without sacrificing any colour, put it aside to dry out. Inspect for inclusions such as pits, cracks, and heavy scratches.

Shape and proceed to the polisher.

Polishing

We polish the opals on a leather pad using tin oxide powder (see image above).

The tin oxide comes as a dry powder, that is mixed with water and a dash methylated spirit. Make the paste viscous. Not too thick and not too thin.

Apply the tin oxide paste to the leather pad with a paint brush.

The most important thing to remember on this station is to keep the leather, powder, and brush clean. If grit gets into any of these components you will not get a polish. Rather, you will get more scratches and be forced to change the leather.

The objective of polishing is to remove scratches. You work the opal on the gemstone getting a ‘pull’ on the lap. Use magnifiers to help see all the scratches. Use plenty of light.

Once the opal has been polished remove it from the dop stick and finish the back of the gemstone on the grinder.

A dop stick is a ‘stick’ upon which you glue the opal to enable it to be held. We use nails and glue the opal with a glue called ‘loctite’. You can get this glue at the hardware. Do not allow the nail to touch the diamond wheels at the grinding station. The metal will damage the diamond grit impregnated into the wheels. Some cutters use dowling and schellac flakes.

Get in touch if you have any questions and would like a specific aspect of the cutting process explained in more detail.

Good luck and happy cutting!