Finishing Boulder Opal

Apr 16, 2020 | From the Work Shop

Poly what?


It’s said that necessity is the mother of invention, a saying that rang true for us when faced with the dilemma of finding a new way to finish the boulder opals we couldn’t finish in the traditional way without losing too much opal from the face and reducing the beauty of the stone.

Back in 2010 we had built up reserves of boulder opals that had great color and character, but if we processed them using the traditional method, that is sanding and polishing, the pieces would lose their color and presence of piece.

The traditional method is the way it’s always been done and involves grinding and sanding fine layers of the opal (and rock) to remove scratches to achieve a high gloss finish.

Boulder opal by nature is relatively thin.

The opal appears in the host rock as a vein of color.

Complex And Beautiful Nothing Compares To Australian Boulder Opal

Complex and beautiful nothing compares to Australian boulder opal

One theory is that 100 million years ago the silica gel flowed into cracks and crevices of large weathered boulders and faults in the ground and over time solidified into OPAL.

Learn more about how opal is formed.

In an ideal world, the vein is reasonably thick thus allowing us to remove the rock from one side of the vein; to face down i.e. expose the opal face.

To do this we use various diamonds grits on grinding wheels, a process that is done under water.

Shaping The Opals On Diamond Grinder In Water Crop

Shaping the opals on diamond grinder in water

Using a grinder with water is a great way to face down to expose the opal ‘face’ however it is impossible to get the perfect finish on the grinder as the water distorts the ability to ‘read’ the surface. By this I mean to see scratches (size and depth) and to identify cracks, pits and various natural inclusions. Under water the opal looks like it’s finished, not an inclusion in sight but when they dry out it’s a different story – all is revealed!

Another reason for not being able to achieve a finish on the grinder is that the diamond grinding wheels can create a wavy effect on the opal face that sometimes harbors fine scratches in the troughs that are hard to see and remove.

The best way to ‘sweep’ these scratches off the opal face is to sand it on wet and dry sandpaper.

To do this we put the opal on a stick (a dop stick) and work it on a disc sander, dry.

Opal On Dop Sticks

Dop sticks help us hold the opal. Some use shellac on dowl, we use nails and glue.

The advantage of dry sanding is that we can see inclusions clearly and work the gemstone accordingly. The disadvantage of dry sanding is that it generates heat. Heat can crack opal, so it is advised to take care when sanding, keeping contact time of the opal on sandpaper to a minimum. To reduce the amount of time the opal is worked on the dry sander we choose our grits carefully. I will write a guide on what grits we use when sanding boulder opals the traditional way in a future blog, but for now we’re focusing on finish.

Wet And Dry Sanding

Once the opal face has been sanded and all scratches removed, it is polished on a leather lap with a tin or cerium oxide paste. The objective of polishing is to remove even the finest scratches and achieve a high gloss finish.

What happens to all the opals that cannot be finished the traditional way?

Good question and one we decided to address in 2010.

As mentioned, we had built up reserves of these pieces, semi processed gemstones that would lose their opal presence and beauty if finished in the traditional way.

That’s when we came up with the Polytop finish.

Rick struck on the idea of finding a product to ‘seal’ the opal, while giving a smooth finish, and after considerable research and development he found a polymer that achieved just that.

He ran with this idea and applied it to the opals (on dop sticks) that had been processed to a point where they just needed finishing.

The opal still needs to be prepared to near the same extent as per the traditional finish. The shapes need to be refined; edges sanded however the opal faces are only lightly sanded.

A Batch Of Semi Processed Pairs Waiting To Be Finished

A batch of semi processed pairs waiting to be finished.

Only then are they ready for the polytop finish. The polytop is only applied to the opal face and sides.

The back of the opal is finished traditionally.

Six attributes of a polytop finish

  1. Conserves the opal presence on the face and protects it.
  2. Enhances its robust nature against knocks, hence promoting its longevity.
  3. Stabilizes fissures, fills voids, pits and natural rough faces giving a smooth, high gloss finish.
  4. Captures the scenic character of an individual opal by showcasing the rock/opal relationship as it appears in nature – mostly untouched by the cutters hand.
  5. The polymer is hard and will take a rigorous level of wear and tear.
  6. The polytop can be rejuvenated easily and cost effectively

Lastly, as all the opals in our range are solid opals (natural gemstones) they can be worn in water. The polytop finish not only presents the opal gemstone at it’s absolute natural best but adds to the lifeline of the opal.

Opal Finish


The finished product!

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