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.