Lapidary: The process of making a cabochon

These two pieces of stone are from a slab of agate and are a custom lapidary order for a lady in Nova Scotia. The lady contacted us at a show and indicated that she had slabbed agate from her late father’s collection that she would like it cut and cabbed to be made into earrings.

The original slab or ‘cutting rough’ was cut from a larger piece of agate rock on a rock saw. It was approximately 3/8 of an inch thick. In turn, we inspected the slab for cracks and problematic inclusions that could jeopardize the integrity of a finished cabochon. We found several cracks at the edges of the slab; which is not uncommon and is one of the biggest reasons for significant wastage in cutting rough. Inspection of the slab in both sunlight and against bright interior lighting is necessary for determining potential problems in the remainder of the cutting process.

Permanent marker (which will polish out) was used to mark the approximate location of the cabs and the slab was cut on our diamond saw and trimmed to closer approximate the desired oval shapes. Further shaping is done on the rough diamond disks when the shape has been drawn onto the stone.

The first set of disks used to shape and start the polishing process are 325, 600 and 1200 grit diamond embedded in steel. The 325 disk is ideal for roughing shape and each subsequent disk allows for greater defining of shape. As with woodworking or auto body, the marks made by the heavy grit must be completely removed by the next grit or a good polish can not be achieved on the stone.

The next stage in crafting a cabochon is to use the to diamond impregnated leather disks in 325, 600, 1200 and 3000 grit. Again any marks in the stone made by the heavier grit must be polished out by the next finer grit, if not, you must go back and repeat the previous polishing step and continue to bring up the shine. For example marks left by the 325 disk should be removed by the 600 disk before moving on to the 1200 disk. If the scratches from the 325 were not removed and are found when polishing with the 1200, you must return to the 600 disk and repolish the stone to remove the 325 marks. If a stone’s polish is in question, colouring it with a permanent ink marker and polishing, will remove the ink on the surface but will leave colour in the scratches left by a heavier grit. In some cases, pitted and porous stones are often treated with fillers, like ‘Opticon’ or polymer resins, in order to enhance the color and to increase the integrity of the stone (filling cracks or pits) so that there is less wastage. This use of fillers, though not always desirable can be practical when working with high priced rough, such as high-grade opal.

Dop sticks are a useful tool. They consist of anything that lapidaries can use, other than his or her fingers to hold a stone in place while shaping and polishing it. There are dop sticks made from dowels with the ends shaped to a calibrated cabochon size, which are useful when cutting very small shapes to manufactured steel dops, with the most practical and readily available being long nails used in house construction and pencils.

The dop itself is a sturdy wax made especially for this purpose that is melted in a small pot, usually electric or even over a tea-light candle, wax is gathered into the end of the stick and quickly applied to the warm stone. This bond will usually withstand most of the polishing process, and usually only lets go if the stone is overheated by the polishing process. This is a good failsafe, as some stones can easily fracture under too much heat.

Other materials used for dop include instant glue and shellac, but they require removal from the stone with acetone, whereas the wax releases easily with some heat and can quickly be scraped off the back of the stone.

Further polishing of the stones can be done on 8000 grit and diamond paste applied to felt buffing wheels is a great way to enhance shine on a precious stone. Pastes range from 14,000 to 50,000 with specialty pastes available in even finer grits.

How much polishing a stone gets will often depend on variables such as the stones hardness, opacity, and the desired overall finish.

Some stones such as the agates, that we are showing in this article, can be left at 3000 or 8000 grit because the hardness of the stone allows for a good reflective finish at these lower grits.

These stones started as a 5x6 inch slab, and after removing the poorer areas of the slab were odd shaped pieces about 2 inches in size, the final ovals are 7/8 x 5/8 inches, a nice size for her earrings

We don’t often do a standard oval cabochon in our cutting as we often cut designer cabs, a term used to describe any non standard cabs that are unlike the ovals so popular in commercial import jewelry lines.

Here are the finished earrings - they have a post back: