How to cut a stone cabochon - Bloodstone Cabachon

A How-To Guide on Cutting Gemstone Cabochons

So how do you make a cabochon out of a rough chunk of rock?

Cutting a cabochon involves cutting and polishing a semi precious gem stone into a polished piece of rock with one flat side and one domed side. It is a fairly simple process once you know what you are doing, but it does take quite a bit of practice to perfect.

This is my quick guide on how to cut a stone cabochon which will hopefully give you some sort of idea on how the process works.

Equipment:

Essentially, the equipment that you will need is a lapidary trim saw (or a converted tile saw) and a cabochon machine. The other bits and pieces that are needed include a template and a scribe to mark out the shape of your cabochon on the stone. You may also want a waterproof apron because of the spray from the machines.

Safety Note: It is also worth noting at this point that precaution needs to be taken when cutting some stones and as a result, safety equipment such as eye protection and face masks may be needed. A couple of examples of this would be Nephrite Jade (which is closely related to asbestos) and Bumblebee Jasper (which has a high sulphur content).

Robilt Trim Saw

This is my trim saw, it is an old Robilt machine that belonged to my Grandfather. I recently had to rebuild the base. It is equipped with a 6 inch diamond blade and has a reservoir of water to keep the blade and stone cool whilst cutting. I also use an anti corrosion agent mixed in with the water to help lubricate the blade and to keep it all from rusting. 

CabKing Cabochon Machine

The cabochon machine I use is called a CabKing 6v3 (www.cabking.us). It is set up with 6 vertical wheels of varying grits and a polishing pad attached to the right hand side. These machines can be purchased via Aussie Sapphire (www.aussiesapphire.com.au) for around $2000.

The other machine that you may need is a slab saw (I have an old Robilt one that also belonged to my Grandfather) but slabs of rock are easy enough to come by without worrying about the expense of a large slab saw, which can become quite costly. Head to any gem show and you will find plenty, or search online.

If you don't like the sound of the $$$$ for the machines, a simple and cheap alternative is to join a local gem or lapidary club who have all the required equipment and plenty of experienced people to teach you the tricks of the trade.

Start up:

Heliotrope (Bloodstone) Rough

Captain Obvious: Start with a chunk of rock!


This is the piece we will work on today, it is a piece of Heliotrope (commonly known as Bloodstone) which originates from India. Heliotrope is a form of chalcedony which gets its nickname from the red inclusions of iron oxide or red jasper which look like blood splatters (see below).





Heliotrope (Bloodstone) Slab

The first step is to slab the chunk of rock into thin slices. It is generally accepted that either 1/4 inch (6.3mm) or 1/2 inch (12.7mm) is the standard thickness for a slab. My preference is for either the 1/4 inch or the 1/2 inch slabs, depending on the size of the cab (the thicker being better for large cabs and thinner better for smaller cabs).
 

The next step is to mark out your preferred cabochon shape and size using a scribe and template

Bloodstone slab marked for cutting

For this example I decided to go with a simple 35mm x 18mm oval standard cabochon. When being taught at a gem club, the simple oval shapes are generally the first that are taught with your first stone most likely being a 30mm x 40mm cabochon.

Basic Shape:

Heliotrope (Bloodstone) Rough 'Preform'

Using the trim saw, roughly cut out the general shape of the cabochon from the slab. It's best to remember that a saw blade is a bucket load cheaper than new wheels for your machine, so the more you take off the stone at this point, the less you need to take off with your diamond wheels.




Now it is time to move on to the cabochon machine. The most important part of the cabochon process is that you need plenty of water running over the diamond wheels. Without water, you will ruin your diamond wheels which can be a very costly mistake!

Metal Bonded Diamond Wheels on a Cabking 6V3

The first three wheels of the machine (pictured to the right) are used to shape the stone into a domed cut stone. The first two wheels are metal bonded diamond grinding wheels, the first is 80 grit and the second is 220 grit. The third wheel is a 280 grit resin bond diamond wheel.





Grind the remainder of the excess material from around where you have marked the shape, remembering to use plenty of water.

Bloodstone Shaped 'Preform'

At this point in the process, many people will attach the stone to what is known as a 'dop stick', which is basically a piece of dowel. You attach the stone to the dop with jewelers wax (some use 2 part epoxy glue, particularly with smaller stones) making sure that the stone is level and the dop is centered. I prefer not to dop, but that is simply a personal preference. I do however need to dop smaller cabs, ones that are too small to handle on their own. It is also common practice at this point to mark the center of the stone (on the top side) and to mark a line around the girdle just above the base of the stone. This helps the cutter maintain a standard height of the girdle around the edge and to help cut towards the middle of the dome. Again, all of this is personal preference.

Creating the Cabochon:

Cutting of Bloodstone Cabochon

Using the diamond grinding wheels, grind away the sharp edges of the stone, starting at a 45 degree angle.
You will end up with a 'flat' face right the way around the stone, and you will also end up with 2 edges instead of one.

Cutting of Bloodstone Cabochon

Follow the same process to remove the 2 new edges.
Keep working on the edges that are created by the grinding wheels by working the stone around the wheel.

Cutting of Bloodstone Cabochon

Eventually you will end up with a domed piece of stone................

Which looks something like this when dry:

Cutting of Bloodstone Cabochon

If you look closely, the white bits on the picture above are actually scratches made by the tiny diamonds in the wheels. You can only really see them when the stone is dry, but with experience you will be able to feel them when the stone is wet and wont need to dry it each time you need to check if they are still there. The next wheel (the 280 grit resin bonded wheel) cleans a lot of these bigger scratches off. In this following picture, you can see that I have run the top half of the stone over that 280 grit wheel:

Cutting of Bloodstone Cabochon

Cutting of Bloodstone Cabochon

The stone after the 280 grit wheel:

As you can see, all of those bigger white scratches have been removed by the 280 grit wheel. If you were to look closely, you would see that the stone is still covered with tiny white scratches, but they are a lot smaller than in the photo above. This is why the stone still looks quite dull.

Pre Polish: 

Resin Bonded Diamond Wheels on the Cabking 6V3

The next three wheels on the machine are for the pre polish. All three are resin bonded wheels with grits (left to right) of 600, 1200 and 3000. Essentially what each wheel does is put finer and finer scratches on the stone to give it a shine, these wheels also smooth off any ridges or edges left by the courser grit wheels. Again, you need plenty of water running over each of the wheels to lubricate.

Cutting of Bloodstone Cabochon

The stone after the 600 grit wheel:

Cutting of Bloodstone Cabochon

After the 1200 grit wheel:

Cutting of Bloodstone Cabochon

 And after the 3000 grit:

Final Polish:

2 Gram Syringe of 14,000 Grit Diamond Paste

For the final polish I use a canvas polishing pad coupled with 14,000 grit diamond paste which is purchased in 2 gram syringes. Many cabbers will follow the previous steps up to 100,000 grit, then polish with cerium oxide paste.

Canvas Polishing Pad attached to end of Cabking 6V3

The pad attaches to the side of the CabKing machine (as seen in the picture to the right). The final polish is the easiest part of the whole process, but if you have missed any scratches in the previous steps, they will stick out like the proverbials after the polish! You simply start the machine and run the whole stone over the polishing pad for a couple of minutes. Water is optional at this point, however if you decide not to use water you need to take care to ensure that the stone doesnt heat up too much as this may cause the stone to crack or break.

Don't forget to put a small beveled edge (on a 45 degree angle) on the back of the cabochon to avoid chipping:

The final product:

Finished Heliotrope (Bloodstone) Cabochon

All that is left to do is take a nice photo of the cab - I am still working on this bit - my wife is a far better photographer than I!

Finished Heliotrope (Bloodstone) Cabochon

Thanks for checking out my blog! I hope that you have got something out of this 'crash course' in cabochon cutting. The above stone is available for purchase through my store here along with plenty of others. Please also check out my faceboook page aswell - www.facebook.com/jakagems

Feel free to leave some feedback, let me know how I can improve this how to guide, or let me know if you have any questions!

Until next time!