Lapidary Fundamentals: Cabochon Cutting

 
Cabochon cutting is the most common style of gem cutting. Don't get fooled by the word "common'. It requires persistence, time, and effort to learn this skill completely. 
If you are starting, it is always good to practice rough materials sold specifically for cutting practice.

Cut sharp edges

 
Cabochon Cutting Machines
Cabochon carving tools can be expensive or cheap, depending on your budget and wallet size. The common cabochon cutting machines are Genie and Lortone.
Genie ­- Genie is the well-known cabochon cutter machine that Diamond Pacific made, it was made with mounted six permanent wheels that include the first two to be 100-100 grit diamonds. 
They were backed up with a foam that adapts to the curvature of the stones. Genie machine was supported with two movable Geyer that provide water as a coolant or lubrication and a flat disk mounted on the end to enable you to change the polishing method whenever you want.
Genie machine is one of the best cabochon cutting machines since it doesn't let you spend a lot of time changing accessories, and its wheels last a long time. 
It isn't portable because it is large, heavy, and expensive.
Lortone - This machine is slightly cheaper than Genie, but it has excellent features that make it an excellent machine. It has an in-built expanding drum, saw, silicon carbide grinding wheel, and a flat disk for polishing.
Rock Rascal - Rock Rascal has been around for a long time, meaning it can last long without wearing out. 
It is the least expensive of the three machines in this article and is made with only one station. This draws you into a position of changing accessories now and then.
 
Cabochon Cutting Techniques
A (Properly Charging Ceramic Laps, n.d.) article stated that there are various techniques to shape cabochons, but the oval one is the commonest. I will be talking about cutting an oval shape.

Slabbing
Slabbing requires a saw that will enable cutting the cabs into 1/4 to 1/3 slices thickness. This enables you to figure out the largest piece of cab you can cut by placing the template over the slab.

 
If, of course, your element is a solid color, it is the very first step you take. If there is a pattern, the most eye arraign will be about size and shape. The more the pattern changes the more you continue to round the top, so it is more of guesswork.

If you have chosen your preferred cutting part, marking it with an aluminum pencil will help you avoid guesswork. Keep in mind that some crystals absorb liquid ink, which can cause harm to the gem, so using the aluminum pencil will prevent such circumstances.

Never underestimate the use of drawing guidelines with an aluminum pencil before you grasp the process properly. Once the marked part is done, take your gem to the saw and cut it to the sketch mark you drew. 
Always use dop sticks for smaller stones; this will save your fingers from unnecessary hurting, and you can continue on the grinding level in case of large stones.
 
Shaping

How to shape gems.
Shaping requires gentle care; always use the coarsest wheel on hard stones like corundum and smooth wheels on soft stones like turquoise etc. 
I know smooth wheels cut a bit slower, but it is vital to be slow and produce something good than getting fast and spoiling everything. That, of course, depends on how you see things.
 
Initializing the Cutting

Start your machine to a speed and get your wheel wet to avoid problems like heating, friction, sparks, etc. For additional lubrication, many manufacturers recommend using an additive. Use additive made of rust preventative elements if only your machine is fully made of stainless steel.

Focus on using the lower quarter of your machine when grinding to avoid your stone from being snatched out of your hands from above the centerline. This breaks the stone and causes harm, such as deep cracks.

Take the help of a pair of calipers as you grind the excess material down towards the sketch mark you drew. You should know relying on templates isn't the best option, but it is not wrong since templates are best for shapes but not accurate for dimensions.

 
Peeing the Apple

Peeling the apple is the next step you take after shaping the outline. You should grind a bevel of about 45° and bring it 2/3rd to 2/4th down the way button. 

Continue with an additional grinding at about 60° and make sure you leave the previous cuts visible. Continue the process (peeling the apple) and increase the cutting of bevels at steeper angles until the center of the slopes is met.

It is the hardest step, but it requires patience to master the tiny area in the center that is missed during cutting, making the cab fail to take a good polish. You should be very careful with those flat areas or poorly domed areas in the center as they are the ones that show the most.

Examine the stone on both sides and ends to ensure the whole stone is domed, and watch it out after every step. Use finer grits for any mistakes, as they are good at correcting where you might have gone wrong.

 
Bezel Setting Preparation
 

Smoothing
If your gem's shape is exactly what you wanted, the next thing will be sanding out the scratches from the coarser and other rough stages. You should use 200, 600, and 1200 grit to complete the next steps.

 Keep inspecting the gem after every step examining the curvature to avoid mistakes. Mistakes such as failing to remove all the coarse scratches will leave you with a complete redo of the whole process or produce a poor-quality winding up.

Always dry your gem to see clear progress or look for a warm environment before starting since a wet or humid environment makes it hard to view any progress. Or, if not, use abundant warm water and many towels to dry out your hands and the stone.

Use 600 grit if you are working with silicon carbide, which eventually becomes a finer grit for pre-polishing, thanks to its faster wearing-out ability. However, it is advisable to use two belts 600, one for shaping and another one for pre-polishing.

Make sure you highly focus on your pre-polishing skills. The surface of your stone should be smooth so that no visible scratching can be detected. Go back to the pre-polishing stage if at all you are having trouble getting a high polish. Remember to use finer grits like 3,000 or 8,000 for much softer gems to get the best results,  (Properly Charging Ceramic Laps, n.d.).

 

Cabochon Polishing

Usually, polishing is confusing, but it does get fun as you continue to gain experience. Most new machines come with built-in polishing systems, which is great for a beginner.

Tricks to Gem-cutting Trade

·  Cerium oxide works best for quartz and opals, but you should be mindful of heat building up and damaging opals during the polishing stage.

· Use aluminum oxide soft side out on suede as this trick will put a high polish on many materials better than those other methods.

· Use chromium oxide on leather or felt to polish Jade as it requires more heat and friction.

Always aim for a mirror-like polish on all your gems as it is the main prior clients or buyers look at to make a choice.

 

Reference:

Properly Charging Ceramic Laps. (n.d.). International Gem Society. Retrieved July 22, 2022, from https://www.gemsociety.org/article/advice-properly-charging-ceramic-laps/