Tourmaline Sensitivity and Directional Hardness

Text by Doug Turet (Introduction by Nancy Attaway in the New Mexico Facetor.)

Many faceters addressed the subject of sensitive
gemstones on Faceters Digest during mid-April. Several
faceters related stories about tourmalines that
broke suddenly when they were being faceted. I mentioned
the times when apatites, chrome diopsides, and
emeralds chipped and even broke while being faceted. I
also shared my experience with two long emerald cut
tourmalines that developed stress cracks after I pre-polished
the pavilions and polished the girdles. I noticed
the cracks when I went to polish the pavilion facets.
These tourmalines had been absolutely water clear. The
stress cracks developed from the crown area and grew
toward the pavilion. My husband, Steve sawed them
into two separate pieces for me to facet later. Texas
Faceting Guru, Charles Covill mentioned on Faceters
Digest that epoxy changes the volume during curing.
He said that epoxy shrinks as it cools. Emerald cuts or
long tourmalines will crack, due to the stress involved.
Too much pressure on tourmaline will cause it to crack.
Facetor Doug Turet of MA supplied a long and
informative discourse on this subject. With his permission,
his post to Faceters Digest is re-printed here:

You are absolutely right, Charles, that epoxies
shrink and cool, but were you aware that they generate
a considerable amount of heat before they cool and
shrink? I do not know enough about the thermal
dynamics of cyanoacrylates to comment about them,
but I do know that they create equally-tremendous
shrinkage stresses upon curing, too. This is why I have
always preferred wax’s elasticity to either of them,
except on cold days, when the waxes can shatter without
much warning. Another trio of often overlooked
stressors are temperature, hand-pressure, and orientation,
relative to the impact of the grit. What follows
refers primarily to long emerald cuts and baguette cuts,
or other such extended length to width ratio designs,
although the principles are certainly applicable to other
shapes used with this material, as well.
Far too often, (from what I have heard), faceters
will begin cutting a newly (wax-) dopped stone, only to
have it shatter on them, or, in the case of bicolors, split
at the color boundary. What is going on when this happens
is two-fold. First, it is important to remember that
tourmaline is both heat sensitive and strongly piezoelectric.
Therefore, any sudden temperature change,
(like the one that takes place when a warm, just-dopped
stone comes in contact with a cold steel lap and a cold
water drip), or any change in pressure against the crystal
is every bit as shocking to its internal structure as a
whack at the tail end with tweezers. Second, because
these crystals have both a differential hardness between
the sides and ends with a marked tendency to develop
lateral fissures, these physical properties must be taken
into consideration before grinding the crystal.
What that all should mean to a facetor is this. If a
facetor plans cutting a long, thin tourmaline “pencil”,
then the facetor should, at the very least, give serious
thought to orienting that crystal at the hardest, toughest
direction (down the crystallographic or “C” axis) and
face that direction into the cutting grain of the oncoming
lap. A facetor should also seriously consider
whether any size of the grit particles that he or she
intends to hurl towards their prize crystal will be large
enough to open a small chip in it. A facetor needs to be
especially wary of a grit particle whose accompanying
subsurface fracturing could possibly lead to one of
those nasty, crystal-length-abbreviating lateral fissures.
For me, there are only two options to consider when
cutting bicolors. Use evenly-worn electroplated laps of
600-grit or finer mesh or use relatively new resinbonded
ones 360-grit or finer.
When confronted with a bicolor, especially one still
in its primary crystal state (i.e. that has not been modified
by alluvial erosion or cobbed into a rounded nodule),
it pays to remember that what makes it unique also
poses a challenge to facet it. The chemicals that act as
coloring agents in bicolors have, by their very presence,
imparted different durabilities with thermoelectric
and piezoelectric sensitivities to each end of the
crystal. The single area of that bicolor rough that cutters
and jewelers alike want most to show to their customers
just happens to be the single weakest part of the
crystal structure. That weakest part is the junction
where these dissimilar elements meet, at the color line.
As such, whenever I am asked to cut bicolors, my
first step is to gently warm the stone. Then, I dop it,
using a wax (preferable a soft one, like green or black).
Next, I soak my cutting laps in fairly warm water to get
them as close (by touch) to the temperature of the
stone. I replace the water in my drip tank with warm
water, as well. The next step in this process is every bit
as crucial as the ones before and after it. I allow a puddle
of the warm water to form on the lap. With the lap
still motionless, I let the stone sit in the puddle for 30
seconds or so to acclimate it to its new temperature. When I do finally touch the stone to the lap, I do so
with the cutting grain as closely parallel to the direction
of crystal growth as possible. I do this so that there is
not even the slightest chance of a lateral crack getting
started from a coarse scratch across the crystal. In other
words, the stone rides on only at right angles to the
outer edge of the lap, often on the outermost one half of
the lap’s surface. It is as if I were cutting the crystal’s
girdle facets but at a steeper angle.
I was first told of these techniques and their underlying
rationale in the mid-1980’s. Back then, I was
lucky enough to meet one of the great master lapidaries
of the last century at a show in New York City. Ever
since learning and putting the above lessons into practice,
I have never lost so much as a single tourmaline
crystal to splitting, not even a bicolor or a tricolor.
Before I overheard him talking about those “rules”,
however, I had only one really good use for most tourmalines.
They were inspirational in teaching me how to
improve my pitching and cursing techniques!
I hoped that I have helped others, just as Mr. Miller
helped me.

All the best,
Douglas Turet, GJ
Lapidary Artist, Designer, and Goldsmith
Turet Design
P.O. Box 162
Arlington, MA 02476
phone: 617-325-5328
fax: 928-222-0815
e-mail: anotherbrightidea@hotmail.com