A Google search found no info suggesting that diamonds were ever
really shaped by cleaving in this way, so I am at a loss to
explain the misconception. This came up again a couple of nights
ago, when I told an aquaintance that I am learning stone cutting.
He was under the same mistaken impression, for pretty much the same
reasons. Does anybody know of any time in history when diamonds
were actually "cut", or is this just a complete fiction?
Noel,
It is not, nor is it today, complete fiction. The problem is that
cleaving is not done to produce finished facets. Well, not today.
Some older rose cuts (flat backs) sometimes appear to have a simple
cleavage plane as the back surface. And some of the really tiny
things called chips (the term is usually incorrect. But not always),
sometimes appear to be just little diamond crystals that have been
split (cleaved) to produce one or two semi flat surfaces. Whether
that’s deliberate, or just as found, I don’t know. But in general,
cleaving is a process used for purposes similar to the way a slab
saw is used in general lapidary work.
Diamond has four directions (planes, if you wish to use the precise
term), along which it is not only at it’s hardest, but along with it
will also tend to separate. cleavage planes are known in other
stones. Topaz, for example, is extremely easy to break in one
particular direction, but only modestly brittle in the others. Mica,
the common mineral, is so cleavable you can separate it into thin
sheets. Not all minerals have cleavage planes. Some have none, and
many that do have them have only one. The direction of cleavage is
dependent on the crystal structure. You don’t get to choose…
Diamond, as it happens, has four such directions. (similar to
fluorite, a very soft mineral which is OFTEN shaped into decorative
rhombahedrons by simply cleaving the stuff and then polishing the
result.)
But back to diamond. The thing is, diamond varies in hardness
depending on the direction through the crystal. In some directions,
it’s softer than others. Diamonds can be facetted using diamond dust
as the abrasive because some percentage of the dust particles will be
presenting a hard enough direction to the stone being cut, to be able
to scratch/abrade it. Diamond cutters need to avoid, if possible,
placing facets parallel to the hardest directions (these are parallel
to the octahedral crystal surfaces), or grinding slows down
tremendously, as the grinding is then done by particles of the same
hardness as the surface being ground. Like sanding wood using
sawdust as the abrasive. Not efficient at all…
Anyway, in many cases, before a diamond crystal is cut/ground and
polished into a finished stone, the crystal needs to be separated
into several parts. Normally, each crystal yields more than one
stone. Now, the softest direction in the diamond crystal is parallel
to the cubic planes of the crystal, and this happens to be the plane
in which normal round brilliants will end up having their table
facet. Well shaped octahedrons are divided into two pieces, either
the same size or one larger than the other, each then being roughly
pyramids. Because this plane of separation is parallel to a cubic
plane in the crystal, it generally is done via a diamond saw. That’s
a thin metal disk with diamond dust at it’s edge, or fed to the edge,
as the abrasive. It was developed in the early 1900s, and is still
in use. Similar to standard lapidary saws.
However, with oddly or irregularly shaped crystals, it sometimes is
desired to divide a crystal in other directions. Some of them, to a
degree, can still be sawn. But if the desired saw cut is parallel to
an octahedral plane in the crystal, the saw has almost no effect.
This is when the longtime traditional process of cleaving comes in.
Another diamond is used to dig a carefully positioned groove on the
surface. A steel wedge is placed in the grove and carefully struck
with a small hammer. It’s risky. Done wrong, the crystal can
shatter, or fracture along the wrong plane. (this is why the
process made a good ad for a smooth riding car) Done right, it just
splits in half. Cleaving is regarded as rather an art form, perhaps
something of a lost art, as the cleaver must consider internal
condition of the stone as well as just the process, to predict how
and where he can do it without accident. One slip with the wedge of
hammer, or the wrong decision as to how and where to attempt to cleave
the stone, and one can destroy the whole thing. Note that the
wedge does not actually cut the way a chisel does. Instead, it puts
pressure on the sides of the grove, pushing them apart, splitting the
diamond the way an axe splits wood along the grain where with a good
hit, a log can sometimes be split in two with only a short penetration
of the axe, but it’s enough to force the wood fibers apart enough to
split the wood. Somewhat the same sort of thing with diamond. And
just like with wood where a knot in the wood can make it split in
something other than a clean flat plane, inclusions and flaws in a
diamond crystal can also disrupt the way the stone cleaves. Thus
the tension and risk. The cutter must hope not only that he’s
correctly analyzed the stone to choose the right places, but then has
to get the process itself, with the wedge and hammer, done just right
too. Some of the large famous diamonds got months of study before
decisions as to where and how to cleave or saw the roughs were made.
Before the invention of the diamond saw, cleaving was the only way
to carefully divide a diamond crystal into multiple pieces of rough
to produce more than one stone from a given rough crystal. One
common reason to do this, by the way, might be damaging inclusions
one would not want within a larger finished stone, or simply a stone
where cutting a single stone would result in much wasted material,
which if separated into a distinct piece of rough rather than just
ground off, can improve the yield. It is, of course, limited in
that only certain directions can be cleaved. With the advent of the
diamond saw, it wasn’t as commonly needed, but the two processes are
not duplicates. Cleaving works only along the four octahedral
directions/planes in the stone, while sawing works everywhere else,
but not along those directions. These days, we also have the recent
development of lasers capable of cutting crystals in half, or
profiles, or whatever, but then the laser cut removes a measurable
width of kerf (so does a saw), while cleaving removes less material,
if any. So even today, some stones need to be cleaved. But it
simply is not the process by which the final shape of the stone is
arrived at.
HTH
Peter Rowe