Pro-Craft Ceramit “enamel” is excellent for this use. As an example,
chrysocolla slabs can be transformed from porous, friable junk into
grand cutting material. This makes gorgeous cabs with the Ceramit
impregnation. With a broad heat-cure cycle, it is very forgiving. A
catalyzed mixture can be stored in the deep freeze where it will stay
active many hours.
Until, fully cured, toluene is the best solvent for cleanup. We use a
heat cycle of 250F for 45 minutes but lapidary hardness will happen at
room temp (after several days). A touch of vacuum impregnation
technique works best for deep fissures. Cut and process the final
matrix with the usual run of lapidary sanding/polishing techniques.
Diamond 100k grit (in silicone oil on a phenolic-fabric belt) polishes
Ceramited chrysocolla to a high brilliance.
Be aware that clear Ceramit in wide seams, looks like quartzy
intrusion. This may not be suitable for your onyx. OTOH, there are
many transparent and opaque colors which are all blendable with the
clear… including a “pearl”. If you care to explore, this can become
a creative art form.
Rio Grande’s “Colores Epoxy Resin” is another system which should be
interesting. My problem there is that it’s only sold in multi-color
"kits", too pricey when you just use a few colors. Besides which,
Ceramit has always worked… why switch? As for “Opticon” epoxy and
it’s stinky amine catalyst, I keep reading about it… but it’s been
only a yellowing mess for me every time I’ve tried it.
Grobet’s catalog #45.848, Transparent Clear, is what I’m referring to,
along with their Ceramit catalyst. It can be diluted with the Ceramit
thinner. Grobet is not the only distributor.