How To Cut A Slab Opal

how to cut a slab opalEvery now and then an opal miner will find such a massive Opal that it takes some care and experience to cut it. This Opal weighs 200 carats (40 grams) and as you can see it is full of sand holes. There are also sections of this Opal that need to be rubbed down to expose more Opal color.

First Step – Study The Opal

It takes years of practice and experience to be able to identify where the opal bars are going inside the stone. The first step is to always study the opal. Try and figure out where the opal colour bar is running and make an estimate of how far below the surface the colour is. This slab opal had no hidden bars and on the side it displayed black potch reaching the surface. It was decided the opal was best to cut with a saw first.

Second Step – Mark It Up

Once you have an idea on how you are going to cut the opal it is best to mark it out with a marker. This is going to make it easier when sawing the opal. This is especially true when talking about seam opal. Seam opal is opal formed in a long slab formation. An experienced cutter will check for colour bars that hidden or are undulating. The black lines are drawn on the surface showing the possibility for 6 clean Opals.

slab opal marked up with black pen

Third Step – Slab it Up

This is the fun part. Jump on the saw and slab it up. Follow your markings and you will instantly start to see what shape your opal is going to take.

When sawing always cut out the smaller opals first and then check that there is not an underlying bar of colour. Cut one stone at time and always double check . Do not cut out the largest opal first just in case the color bar dips.

Fourth Step – Rub it Down

This step is usually called ‘Rubbing Down’ or ‘Pre forming’. It is the rough stage of getting the Opal shape and is usually down just with the Opal in hand. Once you have rubbed a piece of rough opal it is termed as ‘a Rub’.

The opal we are looking at is a 200 carats rub from Lightning Ridge. The Opal Miner just slightly polished the top to show the potential colors in this rub. The specimen weighed 200 carats and was a big sizer (65 x 40 x 9 mm)

final product Black opals

Fifth Step – Final Shape and Polish

The final step is to cut the opal on different grit wheel to get a nice polish on the surface. This is the stage where the opal is put onto a dop stick which helps with accuracy. The opal is shaped and polished to reveal the final opal.

The Finished Product

The large triangular opal cut is 23.25 carats and has two different patterns. It could have been cut into two opals but it is good to see a large black free form shape instead. This one opal would have paid for a parcel and the other 5 opals would be profit for the cutter. This opal is a text book example of a good return on buying a clean rub.

Interestingly that this opal rub was on Opal Auctions for years with over 3,000 views. No one was prepared to gamble on this piece. When cut the 6 opals weighed just under 40 carats so the cost per carat was only $56 per carat for finished polished Black Opals.

Buy Your Own opal Rough

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5 people found this article helpful



Definitely would have carved out the sand and polished as largish specimen as Jewelry is so for posh women.

7th Jul 2016

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