[Jewelry making - Article 00336] - Re: [Orchid] Diamond cutting

[Peter W . Rowe]

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>      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

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