That is true, but also the metal under the stone re-directs
the light that passes through the stone- if the hole were only
slightly smaller than the stone, all that light would be lost.
Jack, to bead-set a gemstone, you obviously need to drill a hole,
the diameter of which is at least as large as the stone's girdle, or
it wouldn't be possible to raise a bead above the stone to hold it
in place. In order to bead-set a stone however, you need a
cone-shaped seat to drop it below the surface of the metal. Sure,
you could drill a straight hole, then set the stone with prongs if
you wanted. But just go ahead and try to keep the stone seated level
on that hole while bending those prongs! Then again, there are
cone-shaped setting tools similar to beading tools that may work.
You also mentioned in your next post that the bur leaves a
"truncated cone." That's essentially true, but since this truncated
cone is so relatively small when compared to the hole itself, you
should really think of it as what setters call it - a seat. This
seat only needs to be as deep as necessary to allow the stone's
girdle to sit flush or near-flush with the top of the hole. That's
what allows you to raise the bead next to the girdle, which allows
the stone to be set.
The bur's angle of 90 degrees (that'd be a hart bur, a setting bur
is normally 45 and there are plenty of others from which to choose)
as compared to a pavilion's angle just doesn't apply because it is
used mainly (but not solely) to cut seats for these applications.
Round burs work better for me with bead settings, as do setting
burs. Hart burs I prefer for channel setting.
And of course it would be unlikely to find a stone with a 90 degree
pavilion or crown angle. Well-cut gemstones are designed with
pavilion and crown angles that are compatible with that particular
stone's critical angle. That is the angle at which light can leave a
gemstone after bouncing around and refracting inside of it.
Technically speaking, little or no light should be redirected back
through a faceted gemstone from the pavilion side. In actuality
though, most faceted cut fall short of the Excellent
category and their visual qualities do change when set with
different colored metals. Transparent to semi-translucent cabochons
are a different story. The point is, the purpose of drilling a hole
for a setting entirely through the metal is, IMHO, twofold; First,
to be able to poke the stone out before final setting, i.e. to make
adjustments, and second, to facilitate cleaning. The critical angle
of a gemstone changes when body oils and grease are present, and
they need to be cleaned from behind to restore their beauty. The
purpose of a seat (truncated cone) is to provide a secure rest for
After all of this rambling, I would like to make a request of you,
Jack. If you do manage to set stones with a straight hole, please
take some photos and post them to the forum. There are some bur
manufacturers who might pay a fortune to keep that info under wraps.
James in SoFL who would like to buy the world a Coke, and keep it