You would be better using a two part clear epoxy polymer. Apparently
the epoxy is superior to the polyester because it has no shrinkage
I am in the middle of making a pendent which will have a piece of
red coral encased in resin. It is actually a more complex job because
red coral is mounted on a piece of reticulated silver sheet using a
dab of epoxy resin and will be embedded in clear resin over the top
of coloured resin. I have made a straight sided silver bezel assembly
fully polished which is filled with two part resin over the top of
the piece of coral.
If you do the job carefully and fill so there is a slight meniscus
above the edge of the bail and don’t move things until the resin has
set then it will require no further polishing. You can burst bubbles
as they rise to the surface by passing a gas lighter flame just above
the surface. Make sure you keep everything clean and the piece
covered with something like a takeaway food container as much as
possible so that dust doesn’t settle on the surface. Also try to work
at a temperature of around 21-25C to ensure the resin sets properly
but you still have a good working time. The two parts have to be
accurately measure and well mixed.
The resin is readily available from art supplies and some craft
stores. Which should also sell suitable dyes you can use to colour
the resin if needed.
All the best