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The stone cutter's art

Hi Gang, I’ve run the saw room for a lapidary club for more years
than I care to remember. In that time I’ve cut 1000s of slabs of all
kind of material from the very best to some that should have been
crushed for road rock.

Most cabs start life as a slab cut from the parent rock. For the
average cab, cut from a non phenomenal, opaque material, the
majority of slabs are cut at 1/4" (6.5 mm) thick. If the material is
rare, sometimes it’s cut at 3/16" (4.75 mm). If the material
chatoyant, or adularescent the thickness may be adjusted to show the
phenomenal properties to the best advantage.

Sometimes ordinary material is cut to different thickness to satisfy
the requirements of a uniquely designed cab.

Many times, the cabs seen at gem shows, especially star rubies &
sapphires, will have an extremely thick base. The thick bases are
generally there for a couple of reasons.

One, since the stones are sold by weight, more base equals more
weight & money. The other reason is one of practicality. Hard
material like corundum takes a while to grind. Since the majority of
these stones are cut from individual crystals, the very minimum of
time is invested in grinding & polishing. Grinding the base off just
adds addition time & cost to the finished stone.

While this doesn’t answer all the questions about why cabs are cut
the way they are, it may give a base from which to start
understanding some of the why’s for the way cabs are cut.

Dave

Derek and all Having just clerked at our local rock & lapidary show
for people judging cabochon cases, I have some knowledge about stone
cutting. You must remember I only just got some cutting equipment
after about 10 years of not cutting so I tried to learn as much as I
could about competition stones.

For Novice competition the stone is of a stated material, size and
shape, determined by the hosting club. The rest of the regulations
are pretty standard in Canada, USA, and Australia. Recommended is
the 30mm x 40mm cab as it can be easily held and seen by both
competitor and judge. The crown must be 20% of the shortest axis
(e.g. 30mm x 40mm stone, 30mm shortest axis and 20% of that is 6mm);
from the girdle of the stone the Bezel should be even, no more than
3mm thick, and polished; the back of the stone must also be polished
with a bevel, also polished, at an angle of 45 degrees to 70 degrees
to the back of the stone.

These of course are the ideals and once you can cut to these
specifications, then you can also cut different stones differently.
Star and Catseye stones for instance are often cut much thicker to
bring out the star/catseye; some lace obsidians or agates are
sometimes thinner to better see the patterns.

Hope this gives another perspective.

Karen Bahr “the Rocklady” (@Rocklady)
K.I.S. Creations
May your gems always sparkle.

Derek

Now, in order to get this bevel you speak of, it’s going to be
necessary to redop the stone. If you try and do this bevel holding
the stone by hand, it’s unlikely to be even all the way around. The
bevel on the back is a very good thing to protect the stone. If the
back is not polished then the bevel need not be even.

Even then, once you’ve got it dopped, because many people want a
perfectly finished back, as a cutter you’re going to spend quite a
lot of time working the back and trying to work around the
imperfections you were avoiding in the first place. I don’t know
about other cutters, but I find that polishing a part of the stone
that no one will ever see once its set feels like a waste of time and
certainly increases my cost of production. Of course there are many
stones in which the back will show, like anything translucent, but
there are very many in which it won’t. As I mentioned earlier, there
are imperfections of lots of kinds on the back. Those setters who
demand a perfect back on their cabs are often asking for far more
than they want to pay for.

I agree. If you know the stone will have an open back, a polished
back is better. I set a lot of larger agates, jaspers, and petrified
woods that I give an open back on so as to show more of the beautiful
stone as the back is often near as nice as the top. Also translucent
stones benefit by extra light that may com in the backs or reflect
off the bottom of the setting and go back out the top. Most other
stones do not need the extra polishing on the back as long as it is
sanded flat. I expect to pay more for a stone that is polished on the
back.

Of course these insights come from having cut some stones myself.
Some stone have not got a nice back anyway, so as long as they are
sanded flat and a slight bevel (uneven is okay) is there to protect
it, the back will be covered by a backing anyway.

Just my 2 cents. [1.7 cents American ;)]
Karen Bahr “the Rocklady” (@Rocklady)
K.I.S. Creations
May your gems always sparkle.