Filling an Empty Space and Enhancing a Cabochon

The vug looks empty after grinding and polishing the cab.

By Bob Rush

I often look for a special feature within a slab when choosing the shape to cut into a cab. When the slab has a uniquely shaped opening caused by the method of infilling in the rough, I always try to incorporate it into the cabochon, especially when it will enhance the piece.

The plume agate pictured had a very interesting vug with a botryoidal inner surface. Finding these features when you are slabbing a particular rock is often an accidental occurrence, but it is a plus. I designed the cab so that the vug was positioned horizontally near the bottom, and it ended up well suited for my cab.

The view after recessing the area where the fire agate would be mounted.

Obviously, wearing this cab as a pendant would not be the best choice because the background, whatever it is, would detract from the cab. Also, the cab’s translucency detracts from its potential beauty, so I decided to install a backing on the cab. My preferred backing is basanite, a black basalt. It is quite durable and takes a great polish.

Even after selecting and polishing the basanite, there was still a void in the piece’s lower right side. I decided to embed a small round stone to fill the void. I struggled a bit in selecting the material to use, as I wanted something to enhance the cab. Then I remembered I had a few small fire agate pieces that might do the trick, but many of the pieces are very directional in their color pattern. I needed a piece that would exhibit its best color when it was hanging as a pendant. This necessity narrowed the choice of available pieces to just one. As I was grinding and polishing the piece, I rotated it under the light to make sure it would show well in all orientations. It did have some slight issues regarding orientation, but I made sure to mount it in the ideal position.

The fire agate fit well in the open space of the vug.

I used diamond wheels to recess the basanite area, where I intended to mount the fire agate. The basanite layer on the back of the cab is 3mm thick, so I recessed the stone 1-1/2mm. I wanted this recess to ensure the durability of the adhesion on the fire agate cab.

As I was applying the epoxy 330 adhesive, I had to be careful to keep it out of the “window” area of the cab so it wouldn’t detract from the beauty of the stone. I think the final appearance has a Halloween jack-o-lantern look to it.

Author: Bob Rush

BobRush Bob Rush has worked in lapidary since 1958 and metal work and jewelry since 1972.
He teaches at clubs and Modesto Junior College. Contact him at

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