Ask any jeweler to list
the gemstones with which they prefer to work, and sapphires and rubies
would undoubtedly rank near the top. With a hardness of 9 on the Mohs
scale, these popular varieties of corundum have long been considered nearly
impossible to damage, and they can easily withstand the melting point
of even high karat gold solders (about 1,6400F to 1,7400F).
Not surprisingly, many jewelers feel a comforting sense of security when
faced with a ruby- or sapphire-set piece. Which may be a problem.
Given the gem's reputation for toughness, jewelers sometimes take the
strength of a sapphire or ruby for granted. It's widely accepted, for
example, that corundum is one of the few stones other than diamond able
to withstand the heat used to retip a prong or perform stone-in-place
casting. And that's true; many rubies and sapphires undergo these processes
with little or no noticeable effect. However, if a piece of jewelry is
subjected to a torch and then rapidly quenched in pickle - a routine followed
by hundreds of bench jewelers every day - corundum can crack like an ice
cube, and so too can its aura of invincibility.
Nor is corundum able to withstand all temperatures. Jewelers must take
care not to overheat corundum - or overlook a possible glass filling,
or work near it with the wrong abrasive: Any of these steps could lead
to embarrassing and costly mishaps. In our more than 20 years as jewelers
and appraisers, we've seen such incidents happen all too frequently. Fortunately,
they can be minimized with just a few simple precautions. By taking even
a small amount of care, jewelers can prevent a broken or damaged stone
and maintain a sparkling reputation.
Before the Bench Prior to picking up a torch or a file, jewelers should first learn a
little bit about corundum and its characteristics. As we've noted, corundum
offers a comfortable "safety zone" for most manufacturing operations.
Part of the reason for this is that it lacks the delicate cleavage of
some other popular gemstones. However, this doesn't mean it can't split
apart; another of the stone's characteristics can offset this advantage
and lead to what is commonly referred to as "parting."
Parting results from "lamellar twinning," a process in which
two crystals grow together in the same plane. This twinning occurs quite
often in sapphires and rubies, and it unfortunately becomes as much of
a weak point as cleavage planes do in other gemstones: If enough force
is applied, the twinned area may de-laminate or part. This problem occurs
mostly in star rubies and star sapphires, since the cabochon cut tends
to expose thin areas of lamellar twinning planes at the top or rounded
bottom of the stone.
However, parting can happen to corundum of any type, regardless of cut.
To minimize this risk, jewelers should be especially careful when faced
with jobs that may require excessive pressure, such as setting a heavy
bezel. In such instances, they should take care to anneal the metal and
to carefully cut the seat to match the shape of the stone.
Jewelers should also examine how the stone has been shaped. As with the
star sapphires and rubies, the cut can sometimes pose more difficulty
than the material itself. We've all seen rubies and sapphires that are
fashioned with pavilions shaped like bathtubs. These rotund shapes can
make setting difficult, since more metal must be ground out of the prongs
to accommodate the bulging pavilion. Consequently, jewelers will often
over-burr the prongs, making them thinner and more likely to break or
otherwise fail. Using heavyweight prongs and slightly larger heads can
help alleviate this problem; as a rule of thumb, the additional burring
should take away no more than 50 percent of the prong's thickness.
Of course, some cuts are too unwieldy to allow any type of neat setting.
Most sapphires and rubies will have a strange, bulky shape when they're
sold. In the past, many jewelers believed they were cut this way simply
to meet the laws of countries that forbid rough from being exported, or
to maximize weight. However, there's also another possible reason: These
cuts may conform to the zoning of color in the stone.
In sapphire and ruby, concentrations of color don't always appear uniformly
from one stone to the next. Consequently, a jeweler who recuts a gem to
fit a specific setting might do more harm than good: If some of those
concentrations rest near the surface, the modification might actually
remove color and reduce the stone's value. While this isn't usually a
problem, jewelers should think twice before indiscriminately recutting
a sapphire or ruby. One simple test that could help is to place the gem,
table down, in a glass of water; the water's refraction will emphasize
the color saturation and provide a guide for "safe" cutting.
If You Can't Stand the Heat...
|A ruby marquise prior to heating with a jeweler's torch
(left) and after excessive direct heat has been applied. Note the
two small white crystals in the center of the stone have "exploded,"
causing the stone to fracture.
|The ruby (left) and sapphire (right) pictured above
both exhibit the results of excessive heat in combination with firecoat,
in which the firecoat has formed circular areas of film on each stone's
surface. The sapphire also has a metallic residue on the film, the
result of scraping with a steel probe.
Obviously, the cut and characteristics of corundum can
influence a jeweler's success. However, most inherent limitations or obstacles
can be overcome by a careful approach to the job at hand-which can sometimes
be much harder than it sounds. As we noted earlier, corundum's deserved
reputation for hardness can sometimes lull jewelers into a false sense
of security. Consequently, they may begin ignoring potential problems,
such as the effects of heat.
While sapphires and rubies can be damaged in various ways during the
repair or creation of jewelry, heat is the most common culprit. It's not
that corundum is sensitive to heat, but that overconfident jewelers can
sometimes mishandle the heat's application.
Part of this can be the result of routine. For many jewelers, the practice
of "heat then quench" becomes reflexive after a while. However,
reflex in this instance can pose a problem. Most jewelers will usually
heat a piece with the corundum still set-and whether performing torch
work or casting stones in place, jewelers who heat and then rapidly quench
a sapphire or ruby run a great risk. We've been called on several times
to examine corundum with one large, visible crack and several spider-web-like
fissures. When asked for an explanation, we've had to explain that these
once-beautiful stones had been damaged by thermal shock-an experience
similar to that of dropping an ice cube in water and hearing it crack
due to the sudden temperature change.
We've seen corundum quenched at the bench with no ill effects many times,
but we've also seen it crack often enough to know that the practice is
not advisable. Instead, allow corundum-set jewelry to cool before pickling;
quenching in this way is perfectly acceptable, and has not been known
to cause damage. In fact, as a matter of course, jewelers should remove
or cover the pickle pot when working with sapphires or rubies; old habits
die hard, and reflexes can always reassert themselves.
Not all problems can be blamed on routine, though. For example, yellow
sapphires can lose color when heated (although sometimes that color may
return). To be safe, jewelers should take any yellow sapphire out of its
setting before beginning a repair. If that isn't possible, coat the entire
stone with a heat-retardant gel. They may also submerge the mounted stone
in water, depending on where the repair is needed.
Problems can also result simply from bad techniques. A bench jeweler
once complained to us that whenever she used firecoat on a ruby or sapphire
during retipping, an unremovable film formed on the stone's surface. As
we thought back, we recalled seeing several stones over the past few years
with the same type of problem. Although the film could sometimes be scratched
away with a steel probe, it was otherwise nearly impossible to remove,
short of repolishing.
Examining a prong setting with a film-coated sapphire, we noticed that
the retips themselves were pitted as though they had been overheated.
A little experimentation seemed in order, so we retipped rubies and sapphires
with several types of gas torches-propane/oxygen, natural gas/oxygen,
and acetylene/oxygen. We also tried a water welder with a self-fluxing
feature, in which the unit actually breaks down distilled water into hydrogen
and oxygen, then sends the mixed gas through a fluxing solution of methyl
alcohol and boric acid. With each setup we intentionally overheated the
Our findings: When the prongs overheated to the point of pitting, the
firecoat did indeed form a film that could not be removed. (Our theory
is that the heat actually cooks the borax into the surface of the stone,
but we didn't have the electron microscope to verify this.) We also noted
that the pinpoint flame and self-fluxing action of some water welders
made it easier to avoid overheating, although the flame burned very hot-which
could cause a problem in unskilled hands.
Ultimately, though, the key to avoiding an ugly film does not lie in
changing torches, but in learning good technique. For example, jewelers
shouldn't use excessive borax in the firecoat, since the borax may "clump"
on the stone. They should also focus the flame on the prong tip, so as
not to heat the stone excessively. Such practices-as well as a better
understanding of corundum's limitations-will lead to fewer problems and
more durable, attractive retips.
Treating Corundum Well
When working with sapphires and rubies, jewelers also
must be aware of the various treatments used to enhance corundum. While
not all such treatments will pose a threat, some may-and the smart jeweler
will be on the lookout for them.
First, nearly all natural rubies and sapphires are heat treated. While
the devices used to accomplish this may vary-they can range from crude
metal buckets to sophisticated computer-controlled furnaces-the purpose
remains the same: to improve a stone's color or clarity though heating
and subsequent cooling in a controlled environment. The specific temperature,
environment (either oxygen rich or oxygen depleted), and length of treatment
all depend on which of those goals is being sought, as well as on the
condition of the original stones.
Heat treating is usually a benign method of improving corundum's appearance,
but it will cause some structural changes. During treatment, voids and
other inclusions in the stones will expand and contract at different rates
from the surrounding corundum, and this can often cause cracks in the
stones. Consequently, many heat-treated sapphires and rubies have "exploded"
crystals. These crystals are usually contained within the stones and are
non-damaging. (If material is damaged, it is generally either recut-if
salvageable-or discarded.) Moreover, the heat from a torch rarely will
cause any further cracking
Actually, jewelers will have the most problems with corundum that has
not been heat treated. The unenhanced rubies and sapphires are usually
the rarest and most costly; many of these have documentation from gemological
laboratories attesting to their untreated condition, and buyers will pay
a premium for them. But because they have not been treated, they may have
inclusions that could "explode" when heated during a repair.
This could lead to severe loss in value for the stone-and a possible lawsuit
for the jeweler.
The best way to avoid accidental damage to these untreated stones is
to learn how to recognize the types of inclusions found in them, and then
avoid any torchwork. When viewed through a microscope, untreated stones
might display "negative crystals" (i.e., voids that resemble
a solid crystal), undissolved rutile needles, or crystalline deposits
of other minerals (similar to negative crystals). If in doubt about whether
such inclusions exist, jewelers should take an untreated stone out of
the mounting or coat it with a heat-shield before the repair begins. (They
may also submerge the mounted stone in water, depending on where the repair
is needed.) If none of these options is possible, the owner of the jewelry
piece should be fully informed of all risks before any work is performed,
and the jeweler should make clear for what he or she will take responsibility.
We strongly recommend that every jeweler have a written policy that they
discuss point by point with the client, and that they have the client
Diffusion treatment is another method that adds color to corundum-usually
sapphire, although diffusion-treated ruby can be found on the market in
small amounts. The treatment consists of packing colorless or light-hued
faceted sapphires or rubies in powdered chemicals that naturally cause
the color of corundum; a blue sapphire, for example, might be packed in
titanium oxide. These chemicals are heated at temperatures of about 2,0000C, until they melt into the stone. The surface is then repolished to eliminate
the ripples caused by the high temperatures, until only a thin crust of
color is left.
Diffusion-treated stones generally remain stable when exposed to heat.
However, any ruby or sapphire exposed to this treatment will now have
a surface color different from that of the interior. This means that any
chips or nicks caused during setting or finishing will be highly noticeable,
and jewelers won't be able to simply polish them out. For this reason,
every sapphire or ruby should be illuminated from behind with diffused
light, and the jeweler should look for the telltale areas of darker color
along facet junctions. This will allow the jeweler not only to take extra
precautions, but also to provide full disclosure to the client.
Then there's the matter of fracture filling. In recent years, the number
of sapphires and rubies with glass-filled cavities and fractures has surged.
While some analysts debate as to whether this is an intentional practice
or the result of packing the gems in borax during heat treatment, everyone
agrees that any gem with a glass filling must be handled extremely carefully.
Since the melting point of glass is typically lower than that of solder
used in prong work, heating such stones will result in the filler oozing
out of fractures or disappearing from cavities.
The best way to avoid explaining why a ruby or sapphire now looks like
Swiss cheese is to learn how to recognize the treatment. To spot a glass-filled
cavity, examine the stone under magnification, tilting it back and forth
to catch reflected light. Glass is softer than corundum and thus does
not take as high a polish. The difference in the quality of the luster
will be readily apparent.
Glass-filled fractures may also show gas bubbles trapped in the glass.
Once a glass-filled stone is detected, it can then be removed from its
setting before any work is done, or be protected with water or a heat-retardant
product. If the presence of glass cannot be ruled out for certain, treat
a stone as though it were filled.
The Unkindest Cut
As we've noted, corundum is definitely hard. Nonetheless, it is not impregnable-as
anyone who carelessly uses abrasives and files will discover. For this
reason, jewelers who find themselves in the home stretch of a project
can't become overconfident: Sapphires and rubies stand just a good chance
of being damaged during the finishing process as they do at any other
Most often such damage is the result of finishing prongs with either
files or cup burrs. Since these tools are tough enough (and usually used
with enough impact) to damage diamonds, they are even more capable of
scratching and abrading corundum. Jewelers should always take care not
to hit the stone with a file edge or burr. In fact, since one face of
the file will always rest against the stone, the teeth on this face should
be ground away. This practice greatly reduces the occurrence of abraded
facet junctions and scratched stones.
Files and cups burrs aren't the only dangers lurking in the finishing
process; diamond abrasives also pose a significant threat. Since diamond
is harder than corundum, these abrasives need only a little pressure to
cause an ugly surface scratch. Given that more and more diamond-impregnated
burrs, discs, and wheels seem to be entering the industry of late, jewelers
must always note the type of abrasive that is being used. This is especially
true of pre-charged wheels and sanding disks. Keeping a sapphire or ruby
safe may be as simple as reading a product label.
However, even if files have been ground smooth and all diamond abrasives
have been banished from the bench, jewelers still aren't safe. That's
because many of the abrasives used in jewelry manufacturing are actually
charged with corundum, and the old adage that "diamond scratches
diamond" is just as true for sapphire and ruby. Sanding disks, rubber
wheels, stone wheels, separating disks, sanding papers-any of these might
contain corundum or silicon carbide (carborundum). Also, be aware that
corundum can go by various names: aluminum oxide, emery, even such brand
names as Cratex and Adalox. Be sure to always read the abrasive's label;
if in doubt, check with the supplier. Of course, any abrasive applied with an excessive amount of force can
damage a ruby or sapphire. In the end, it all comes down to care, and
to respecting the limits of the stone. If they keep that in mind, jewelers
will be better able to keep their sapphires and rubies-as well as their