I have a Huebler that obviously, a previous owner had put a green gem in the butt cap.
I think that any cue maker that is experienced with inlays could do this for you without starting from scratch Ray.
I will go out on a limb a bit and add that there are some custom silver smiths and jewelers that could probably do the job for you as well.
I have an old Harley ring that has wings on the sides. It was missing a chunk of wing. I took it to a silver smith, and not only did he fill in the missing piece, he also carved the shape of the wing back in.
We have a couple of custom jewelers in Regina Ray and I would trust them with the job.
Putting one into a hard surface like a butt cap would be easy for them. I would also trust them to drill into the finish just enuff to put a gem in also.
After all, that is what they are trained to do.
I am currently Gemless myself as my old wedding band and related jewelery ended up at a Pawn Shop.
My birthstone is a Diamond but not big on them, but was thinking of other color gems myself that might lend themselves well to trying it out on one of my cues also. Something Blue or Black.
I think in the proper place or right design, a gem in a cue would look great.