I think that George (correct me if I’m wrong) meant a heliocoil
- machine shops/mechanics use these things to repair damaged
threads. Basically it is a piece of steel wire of the same pitch
as the thread wound in a spring like fashion and formed to give
the correct shape for a standard thread (vee shaped). In a thread
repair the damaged ones are drilled out and then re-tapped to the
diameter of the outside of the heliocoil (the inside of the
heliocoil is the size of the damaged thread). A special tool is
used to insert the heliocoil (a healthy dose of Loctite as well
usually). They can be used in a blind hole as well.
Where they would be useful for settings is that one could be
epoxied into place in the stone to give a threaded rod attached
to the setting a place to call home.
One method not discussed yet is electroplating the inside of the
hole in the stone to get a metal base to form the setting from.
You can build a pretty healthy layer of metal up pretty quickly
on rocks (I have experimented with this as a means to putting a
inexpensive copper bezel on small agate slabs - worked great
except the plating solution etched the agate). Just a though on a
another way of approaching the problem.
On this topic as well - unless you plan on drilling a lot of
holes in rock a diamond core bit (the larger diameters) is a
pricey investment. A cheaper method although slow and very boring
(pardon the pun) is to use a piece of tubing - steel, brass or
copper, in a drill press using silicon carbide grit slurry. Based
on pratical experience - a 3/4" hole 1" deep in nephrite jade
took about 4 hours of standing at the drill press. The hole will
have a slight taper shape to it as the pipe will wear a bit more
on the end. Diamond grit may speed the process up but costs more.
If the hole is small you can also use a flexshaft with a diamond
burr (and lots of water) but getting it accurately round may be
fun. Best thing to do is find a willing local Lapidary. We enjoy
(?) doing this sort of thing.
Island Gem and Rock