Cabochon Making 101
Cabbing with Tucson's Old Pueblo Lapidary Club
Article and Images by Bob Keller

While I have been involved with mineral collecting for some time, I've just recently begun to explore the world of lapidary and its attractions as a hobby. I hate to admit it, but somewhere along the line I had picked up a bit of an elitist "attitude" towards lapidary and the poor, misguided, rock grinding, "grunt" rockhounds, who had weird and peculiar ideas about improving rocks by cutting them up and grinding and polishing on them. I was a mineral collector, not a rock butcher! I guess I figured if God had wanted man to mess with his rocks we'd have been given a lot harder fingernails...

However, it would seem that God gave us carbon in various forms and the brains with which to develop technology. I guess it was inevitable that brains + diamonds = lapidary tools. Well, I've finally seen the light and been converted through my involvement with Tucson's venerable Old Pueblo Lapidary Club. In addition to their ongoing faceting, lapidary and smithing classes, OPLC conducts open shop sessions, giving the members access to and use of the outstanding club workshops and equipment. I've been having so much fun making cabochons there that I thought I'd bring some of my fellow mineral collectors along for a session and share a little of what I'm learning and enjoying.

Here's some views of one of our shops. This room houses OPLC's lapidary shop and contains equipment and tools used in cabochon making such as diamonds saws, diamond grinders, carbide belted sanders, and polishers. Other rooms in this club owned building house our metal smithing and jewelry shop, the faceting shop, and a saw room.

I've decided I need a couple of bola ties as accouterments to my wardrobe. Of course, since my conversion to a rock grinding grunt rockhound, I had to make the cabs for these myself. As you can see, OPLC is very facilitating for members with such inclinations and aspirations.

After finally accepting that a 200 carat faceted gemstone was likely to be regarded by many as a bit too gaudy for a tasteful bola, I selected this slab of chalcedony as a promising piece of rough for this project. I'm saving the 200 carat gemstone idea for a future one. ;) I'd like to say I field collected this rough as I was told by another club member that it was a slab of a Brazilian agate, but the fact of the matter is I found it in a box of scrap, discarded and donated material under one of the workbenches in the lapidary shop. There's enough material in that box to make many dozens of cabs. You don't even need to bring your own rough to get started cabbing with OPLC!

Here's a few of the templates available in the lapidary shop for marking out cabochons with various standard and not so standard shapes. If you're going to marry the cab to a commercial finding designed to hold a standard sized stone, it's important to accurately cut it to a specific outline so it will fit. I'm not planning to use a finding to mount this cab, or if I do use one it will be custom made, which allows me to use a free form design for the outline. After studying the rough for a bit, I decided the shape of another freeform cab I had done was suitable and simply traced around it to establish the outline. The rough was just a little short in one corner, so it gets rounded a little more. It's an art, you know...

The next step is cutting the slab close to the desired outline with a diamond trim saw. On the saw table are both oil and water lubricated trim saws, the water saw being reserved for softer and porous materials which could be stained or discolored by oil. The slab is about a quarter of an inch thick. I would have preferred it to be just a wee bit thicker for a cab this size, but hey, the price was right.

I cut away the material surrounding the outline with a series of cuts on the oil lubed saw. This is messy business and relatively slow going with a feed rate of about an inch a minute. The metal blade that the diamond abrasive is bonded to is soft and easy to dork up if you're too aggressive with the feed rate or try to cut curves with it. However, it's still much faster and more economic to remove material with the saw than a diamond grinder, and I've learned a little extra time and attention spent here carefully cutting close to the outline with successive and progressive cuts is rewarded handsomely during the subsequent outline grinding operation.

After sawing, the cab and hands are both cleaned in a tub of oil absorbent media (commonly known as kitty litter), and then the cab is scrubbed clean of oil with an old tooth brush, soap and water.

The next step in the process is to establish a smooth and accurate outline with the grinder, removing any margins or small corners of material left over from the sawing operation. An 8 inch water cooled grinder with 100 grit diamond wheels is used for this as well as the subsequent basic shaping processes of doming and rounding the cab. Note the plumbing to supply the cooling water and the drain pipe to carry away the runoff. An ultimate embellishment would be a water heater inline to warm the incoming water so it wouldn't be as cold on the pinkies...

Because this cab has straight sides and a relatively large area to hold on to, I elected to shape the outline on the grinder prior to dopping it to a dop stick. A dop stick is used to hold and manipulate the cab during grinding, sanding and polishing operations. They're pretty handy to have mounted at this stage when you're making a round or oval shaped cab, and especially if it's of a small size.

A tapered outline, narrower on the face side than the back side, can be established at this point if the cab is to be prong or bezel mounted in a finding. It's also a quick matter to apply a slight bevel with the diamond wheel to the edge on the back side of the cab to help prevent chipping if the edges are going to be exposed.

Once a smooth outline and any desired bevels are applied with the diamond grinder, a reference mark is established around the perimeter of the cab. I marked this line at about two thirds of the slab's thickness and closest to the back side of the cab. This line is used to help judge the progress and facilitate uniform removal of material during the shaping process.

Prior to dopping the cab, it is placed on top of a wax heater to warm so the dopping wax will bond to it readily. It's important that the stone be clean and up to heat to get a secure bond. If this bond breaks while a cab is being worked, it's possible for it to be thrown by a rotary device and possibly broken or irreparably marred. It's also possible that you or someone else could get struck by a flying rock. Needless to say, the prudent lapidary wears eye protection while working with these tools.

The wax heaters are simply metal boxes with a light bulb inside for the heat source. These would be an easy home brew. Believe me, they get plenty hot. After the stone has heated up, a glob of dop wax is picked up and swirled on the dop stick, which is just a section of dowel about 6 inches long. It's then plunked down on the backside of the cab, and a finger can be employed to work the wax into a nice fillet.

The cab and dop stick are left on the heater for a few more minutes, giving the wax time to flow and bond. Then the assembly is removed and allowed to cool. The bond is then tested to make sure the cab is securely bonded to the stick. Once satisfied that all is properly prepared, the next step is grinding the face of the cab to a dome shape and generally rounding and smoothing it.

Sometimes there's rockhound traffic jams on frequently used equipment like the diamond grinders in the lapidary shop during busy sessions, but the congestion normally clears after several minutes as members move on to subsequent stages on their projects. I've learned to bring the material for several projects so that if a particular station that I need to use is crowded or full, I have something else to work on. That way I can stay as productive as I want.

I think one of the things that's appealing about cabochons is they're quick to make. I can often start and finish two complete cabochons, slab to polished cab, in the course of a three hour open shop session at the club. It feels like I got something accomplished when I leave the club at the end of a session with a couple of new cabs in my pocket. That is quite a contrast to my project progress in a weekly faceting class that I also take through OPLC, where I have literally spent months to produce a single gemstone.

Cabochon Making 101 - Part II

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Bob Keller