Introduction to Cabochons

A cabochon is a smoothly convex, unfacetted gemstone. They are often oval-shaped, but can also be square, a heart, freeform, Texas-shaped… basically anything as long as it is rounded and polished on the top. The picture below shows are oval cabochon of amber containing an insect.

Making cabochons was one of the first ways man had to improve the appearance of gemstones. It does not require many complex facets at fixed angles from each other. All one needs to make a cabochon is lapidary rough (the ‘gemstone’) and a series of ever-finer grits to shape and polish the stone.

The University of Texas is unique in its ability to offer a gemology class to its students. Students in GEO 347K have access to faceting equipment during the semester, as well as access to Room 6 (the ‘Cab Lab’) in the basement of the Geology Building. The Cab Lab has everything a student in GEO 347K needs to make cabochons. Diamond saws, grinding wheels, and polishing wheels will all be used in the process of bringing a slab of rough into the finished cabochon.

Step 1 – Pick out your material

The first step in making a cabochon is picking out the rough. The table in the SE corner of room 221 - the lab room - has many types of rough at very reasonable costs (all less then $1). The rough you use will need to be in slabs like the material in 221, or you will need to contact your TA about whether the rough you bring in on your own is suitable. There must be a flat back, and the rough must be thick enough. The material can’t be too porous or soft, either, so if you have any questions, consult your TA.

Step 2 – Pick out a design

Pick out a design for your cabochon. The second desk drawer in the Cab Lab has a number of stencils in it. If you want to purchase a setting for the cabochon at a later date, it is best that you use these stencils because they will provide you with standard sizes for ovals, circles, squares, hearts, etc. Use the brass markers to outline the stencil. Make sure you place the stencil over an area of the rough that either makes the best use of available material, or has a pattern in it that you like best. You can make multiples cabochons out of one piece of rough!

Step 3 – Trim with diamond saw

There are three diamond rock trimming saws on the table in the S end of the Cab Lab. There are three gray switches below these saws that turn each one on. Turn the saw on and make sure there is enough saw oil in it (the saw should be spitting out a little bit of oil from the reservoir below it). If not, contact a TA and use another saw. Most should be just fine, and if your stone gets oily during sawing, then there is plenty of oil in the reservoir. Hold the stone with BOTH HANDS while trimming (the saw has no teeth, just embedded diamonds, so it won’t cut your hands off!). Consult the diagrams below to see how much to saw off. Throw all bits and pieces in the bucket on the saw table.

Step 4 – Dopping

Take your sawed-out section, throw it in the kitty litter on the saw table to absorb any oil, and then wash it off in the sink. Next, plug in the hot plate and wax on the desk if they haven’t already been plugged in. Put your rough on the hot plate with the stenciled-shape facing down. The wax will take ~30 minutes to heat up properly, so you’d be best turning it on in the beginning next time. The goal of dopping is to apply the rough to a wooden dop stick, which will give you something to hold onto during grinding and polishing of the cabochon. Dop sticks are in the first drawer of the desk. Follow the directions above the desk on dopping, and be sure to allow your stone to cool fully before proceeding. Failure to do so will probably result in an unsuccessful dop, and you will have to start over again will your stone flies of the dop stick.

Step 5 - Grinding

This is the most important skill to learn when making cabochons. The goal is to make the surface of the cabochon completely rounded and smooth. The north side of the lab has a series of grinders and wheels, with grit sizes from 100 (coarsest) to 600 (finest). Before using any of these, make sure the water is on! To do this, turn on the spigot behind the machine, then adjust the individual spigots above the wheel you intend to use. OK… so the water is on… what do you have to do to actually make a cabochon? The idea in cabochon making is to grind the top of the dopped rough at various angles to approximate a rounded surface. See the diagram below…

Figure 1 shows a side view of the rough on top of the dop stick. Hold the stone at a ~45 degree angle to a 100 or 220 grit wheel. If it grinds too quickly on the 100 grit, switch to 220. You will form the “First Bevel” fairly quickly. Do this all the way around the stone, but make sure to maintain a small portion at the bottom of the stone flat. This flat area around the stone is used when setting the cabochon. Figure 2 shows three more angles. Develop the 45 degree bevel completely around the stone. After that is completed, and the flat area on the bottom is of equal thickness everywhere, proceed in making a 30 degree bevel on the edge of the top flat area and the 45 degree bevel. Make an even shallower bevel after the 30 degree one is finished (diagrammed here as 20 degrees).

After all these bevels have been added, your stone will look like the dashed line, with an even, flat base. Figure 3 shows the stone from the top after the 45 degree bevel (A), the 30 degree bevel (B), and the final shallowest bevel (C). Make sure if you are doing an oval that the flat area in the center while beveling is centered and oval shaped as well after each step!

After beveling, you will have a few flat areas, some edges, and deep scratches on top of your cabochon. You must use the 400 and 600 grit wheels to perfect your stone. Use the 400 wheel to take out all edges and to make sure you have a good, rounded dome. Look at your stones profile from both sides. Does it dome near the center in both views? If not, use the 400 wheel to correct this. You cabochon should look like the one below…

Once the dome looks good, and edges and gone, move to the 600 wheel to complete the rounding and take deep scratches out. Doubtless the 100 and 220 wheels have gouged your material, so it may be necessary to use the 600 wheel for quite some time to get all scratches out. Dry the stone and look at it in the light to see if there are deep scratches on the surface. These won’t polish. Neither will a large flat area in the center of the stone, so make sure it is good and rounded. Move back a wheel or two if you have to.

Step 6 - Polishing

If you feel you have a perfect dome, with all deep scratches removed, it is time to polish. Look at the chart by the desk to see what polishing agent you should use. It will probably be cerium oxide on canvas or tin oxide on leather. The leather wheels are adjacent to the grinding wheels, and the canvas wheel is next to the sink. Switch on the wheel, and add just enough cerium or tin oxide (in squirt bottles with water) to dampen the wheel. Polish the flat area at the bottom of the cabochon, as well as the domed top and moving it over the wheel. This should take no more than a few minutes. You will probably see some areas that won’t polish… either due to scratches or flat areas. Move back to the grinding wheels to solve these problems, the polish again.

Your first stone probably will not be perfect, so be sure to consult your TA about any problems you are having so that they do not happen again. After your first couple stones, you should be able to go from dopped stone to finished cabochon is 45 minutes to an hour.

Step 7 – Dop stick and wax removal

Bring your cabochon and dop stick home, and place them in the freezer. 10 minutes in the freezer will make the wax very brittle, so the stone will snap easily off of the stick. Scrape any wax off easily with a butter knife while it is cold…

Make sure:

-To unplug wax and hot plate

-Turn off water spigots

-Leave everything as clean as you found it

-Turn off light in Cab Lab before leaving