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Shaping Boulder opal

May 14, 2020 | From the Work Shop

Just to recap, boulder opal is defined by rock (ironstone/sandstone) hosting veins of opal. The object when cutting is to expose the face of the opal vein within the rock.

So how do we go about that?

Once we have sorted the boulder rough opal into potential cutters it is placed in a plastic container beside the saw.

 My preferred saw runs a 10-inch sintered blade.

Figure 1 – 10 inch drop saw with sintered blade

Figure 2 – 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. The saw has a water feed at the top of the blade and I free hand cut at the bottom of the saw blade. It 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.

Figure 3 – Bench saw with Notched Blade for breaking down large boulders

When the rock is larger than this and with colorful opal veins, I go to the benchtop saw, which runs a notched rim blade, to break it down to smaller pieces and then go back and continue on the sintered blade. The boulder opal rough (beside the saw) to be cut presents in a variety of different ways.

Figure 4 – Notched Blade Close Up

We would have sorted the rough with the careful tapping of hammers along the profile of the opal veins attempting to have the rock cleave apart along the opal veins, exposing 2 faces. In saying that, once again, if gemmy color is evident, we would leave it for the saw for further investigation. Not every piece will cleave apart.

Learn more about sorting raw boulder opal here: https://opalartglobal.com/blog/from-workshop/sorting-boulder-opal/

Returning to my station on the saw, the plastic container beside the saw will have rocks with obvious opal faces and rocks showing an opal vein profile that needs to be explored.

Next, I begin to slice the opal faces from the rock leaving a good few millimeter of rock behind the opal face, how much I leave is determined by the size of the face – proportion and balance. The shape of these slices is very rough and random.

This is where the sintered blade comes to the fore. When looking at a slice, the way the opal presents, re color and pattern, will be the major determining factors of the final shape. We do try and bring symmetry to the shape however we do not sacrifice opal gem color for shape sake. I will saw the slice into roughly the shape and then using the side of the sintered blade I begin to refine the shape. In this situation the blade more finely grinds allowing more accuracy. This often speeds up the whole process as we can move straight to the sanding stage bypassing the diamond grinding machine.

The rocks that have the opal veins profiling are explored using the saw. The objective is to determine how the vein is trapped in the rock. There are usually indications of what the opal may be doing in the rock and using the saw we block it out – a small brick of the rock showing the profile of opal vein on its sides, like a biscuit with the cream in the middle. 

Figure 5 – Drop Saw where first rough shape is done

Figure 6 - Trim saw with 6 inch fine sintered blade for cutting gem grade opal

Once the opal vein has been blocked out, we look at the color to determine which side of the opal vein we want to expose. Once again, I will use the side of sintered saw blade to remove the rock. If the color is bright, I will then put it aside to be processed on the diamond grinding machine.

Getting the final shape is a matter of first addressing the thickness of the stone and then refining the shape, this may be repeated a few times.

Shape is all about balance and proportion.

The Opal Art Global scenic range often features the contrast of the opal and the rock on its face creating unique imagery – nature’s art.

Figure 7 – Grinding Machine

Figure 8 – The Artist At Work

We hope you have found this article helpful. Get in touch if you have any specific questions and we’ll do our best to answer them.

Happy Cutting!

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