Fluxed Up: The Fracture
Healing of Ruby
By Richard W. Hughes
With an appendix by John L. Emmett
In 1997, a colleague and I submitted a paper to a gem industry trade association
for publication in their newsletter. The article discussed a specialized heating
technique used to treat over 95% of the ruby traded in the world market. To our
great surprise, the editor rejected the article, deeming it “too controversial.”  In the words of the official, “many
mainstream journalists read our publication and if they got hold of this story
it could mean big trouble.”
Just what treatment could possibly be so nefarious that it could not even
be discussed in the polite pages of an industry publication? I speak of the use of fluxes to “heal” open
fractures in ruby.
Since that time, the process has been a point of discussion at dozens of meetings
between both traders and gemologists. While some today have a solid knowledge of flux healing, it is
surprising that so many traders and even gemologists do not grasp the treatment method and its impact
on a gem. The following is designed to shine a bit of light into this dark industry corner.
The use of fluxes during heat treatment is not a recent
invention. In the early 1980’s, while I was directing Bangkok’s Asian
Institute of Gemological Sciences (AIGS), a number of Burmese rubies with unusual
characteristics were brought into our lab for testing. Based on the inclusions,
it was clear they originated from the Mogok mines. But unlike the classic Mogok
stones, these rubies showed numerous thick wispy fingerprints. There was also evidence
of high-temperature heat treatment.
Someone had apparently been cooking Mogok ruby. It was equally clear that
the heat-treatment process was healing fractures, either pre-existing, or those produced by the stresses
of the heat treatment itself.
For a brief period, we saw many such stones. Then, just as suddenly as they
appeared, they were gone.
W.F. Eppler (1959) and Edwin Roedder (1962) were among
the first to clearly describe how fingerprints form in gems, a process illustrated
in Figure 1.
At any point after a crystal grows, it may fracture. Given the proper conditions,
that fracture may later heal closed, leaving a scar-like inclusion typically known as a “fingerprint” or “feather.”
|Figure 1. Formation of a fingerprint
The healing of a crack in a crystal, resulting in secondary cavities (‘fingerprint’).
A. A fracture develops during or after the
B. Healing begins. Growth solutions flow into the fracture
and/or the inner walls of the crack are partially dissolved, beginning
the healing process.
C. Healing continues. Dissolved nutrients are re-deposited
on the inner walls of the crack as the healing proceeds.
D. Eventually the fluid-filled cavities become more angular
in shape, turning into fluid-filled negative crystals arranged in a fingerprint
pattern. The fluid that remains behind has been leeched of its nutrients.
These pockets containing exhausted growth solutions are smaller along
the inner edges and bigger near the outer edges of the original crack.
(After Roedder, 1962)
The healing process involves exposure to a combination of heat
and solvents. In the ground, elevated temperatures and solvents produce healing of
fractures via corundum-containing solutions. Dissolved nutrients (solute) may come
from solvents dissolving surrounding crystals, the exterior of the crystal itself,
or the interior walls of the fracture. This dissolved nutrient material then regrows
on the walls of the crack, “healing” it closed. But an internal scar
remains, something we term a “fingerprint”
Eppler (1959) actually reproduced this process in the lab, producing fingerprint
inclusions in Verneuil synthetic rubies by inserting fractured stones into hydrothermal autoclaves.
Upon removal, the fractures had healed, resulting in fingerprint inclusions.
Figure 2. Anatomy of a healed fracture
A well-healed fracture in a sapphire lying roughly parallel to the basal
plane. The healed areas appear dark, while the undigested fluids are
highly reflective. Note that the pattern of healing relates to the underlying
crystal structure, with angles of healed areas following the underlying
crystallographic structure (in this case, at 60/120°). Photo
© Richard W. Hughes
Fill ‘er up – Surface repair
A year later a group of rubies came into the AIGS
lab with suspicious characteristics. In this case, large surface cavities had
been filled with glass (Hughes, 1984). One of these stones is shown in Figure
This process became known as “surface repair” or “glass-infilling.” Its
purpose was to fill unsightly surface cavities, thus allowing larger stones to be cut.
The first of these stones had huge glass-filled cavities. When customers balked,
the gems largely disappeared from the market. But the idea was now in the minds of both dealers and
gemologists – glass might be used to fill surface cavities.
Figure 3. Surface repair (glass infilling)
A large glass-filled surface cavity is clearly visible in this treated
Thai/Cambodian ruby cabochon. The glass has a lower luster than the surrounding
ruby, and in places has started to devitrify (crystallize). The upper
portion of the filling is below the surface of the gem and so was not
touched by the polishing wheel. The circular black dot in the center
is a cavity resulting from a gas bubble cut through during the cutting
© R.W. Hughes
Desperately seeking glass
In the years that followed, gemologists were zealous
in seeking out glass-filled surface cavities, often to the point of flagging
tiny amounts resulting from inclusions that had melted during heat treatment,
fillings that had absolutely no impact on the weight or appearance of a gem.
This situation left dealers scratching their heads. Stones with “accidental” glass
fillings of no consequence were being lumped together with other stones featuring surface cavities
deliberately filled with glass to add weight or hide naturals. Eventually at AIGS, we came to an arrangement.
When we found accidental fillings, if the owner was agreeable we simply removed the glass with hydrofluoric
acid,  thus allowing us to issue a cert without the nasty “glass
Figure 4. Mong Hsu ruby before and
after heat treatment
Mong Hsu ruby before (left) and after (right) heat treatment. This clearly
shows that most Mong Hsu ruby is not a viable gem without heat treatment.
Photo © R.W. Hughes
As for the gems with deliberate glass
fillings, owners wanted no part in removing them since gaping cavities would remain.
Such stones were clearly flagged on ID reports and buyers rejected them. Thus it
was not long before they mostly disappeared from the market.
Enter Mong Hsu
In the early 1990’s, a major discovery of ruby
was made in Burma’s Shan State, at Mong Hsu (pronounced ‘Maing Shu’ by the Shan; ‘Mong Shu’ by the Burmese), northeast of the provincial capital of Taunggyi. So important
were these discoveries that –
within just a few years – Mong Hsu rubies constituted over 95% of the
faceted ruby entering world markets. This remains true to this day.
But Mong Hsu is not Mogok. Before heat treatment,
the Mong Hsu ruby is almost always an ugly duckling.
There are two major problems. The first is dense silk/particle clouds and a strong
purplish color, making most stones look like low-grade, cloudy rhodolite garnet. This is mainly due
to the crystal’s unusual blue cores. Ordinary heat-treatment removes the blue, as well as removing
silk, making the final product a rich, clear red. The market generally accepts such heated stones without
Figure 5. Diagram
of a flux-healed fracture
The mechanism of flux healing of a fracture in corundum.
A. Open fracture/fissure, unhealed.
B. During heat treatment, flux enters the fracture
and dissolves the walls of the crack.
C. During cooling, dissolved corundum
recrystallizes in the crack, thus healing it closed.
The newly crystallized ruby is essentially a synthetic
ruby grown in the crack alone. It contains small pockets
of now-solidified flux glass, along with some trapped
gas pockets. For purposes of this diagram, the surrounding
natural ruby and the synthetic ruby in the crack are
shown in two different colors. In reality, no distinction
can be seen between the surrounding ruby and the newly
grown synthetic ruby.
D. Any flux glass present on the surface can be dissolved away with acid. The
synthetic ruby in the crack is unaffected by the acid,
as is the ruby as a whole. (Illustration © R.W.
Hughes; modified from Hänni, 2001, SSEF)
Figure 6. Flux-healed fracture
Moderate magnification reveals a flux-healed fracture in a Mong Hsu ruby
from Burma. The irregular dark areas are pockets of residual flux, while
the red areas in between are where the once open fracture has healed
shut with microscopic amounts of what is essentially synthetic ruby.
In some places, the flux residue appears transparent. This is an illusion
produced by reflection off the surfaces of the flux pockets. Photo © R.W.
Crack be gone
This is not the case for the second problem. Most Mong
Hsu stones come out of the ground in a heavily fractured state. But Thai burners
are nothing if not ingenious. For years some had used fluxes in conjunction with
their burning. This pow chemie (‘heating
with chemicals’) was supposedly done for a variety of reasons. According
to some, fluxes produced a shine on the surface of the rough after cooking, making
the color look better. Others said it helped create a desired furnace atmosphere.
Some even believed that it helped prevent thermal shock during heating (an idea
which has been discredited; see Emmett, 1999).
When I examined my first parcel of Mong Hsu ruby, the coin dropped.
Those Mogok rubies with their twisted drippy fingerprints from so many years before were early examples
of flux-assisted fracture healing. And with Mong Hsu rubies, burners were taking that treatment to
the next level.
Figure 7. Surface of a flux-healed
A highly magnified photo showing a single facet’s surface in reflected
light where a fracture breaks the surface in a flux-healed Mong Hsu ruby.
The dotted red line shows the path of what was once an open fracture, displaced
slightly to the right so you can see surface detail. The irregular black
areas are surface cavities where bubbles in the flux were cut through,
while the irregular gray areas are residual flux glass that has been polished.
Note the lower luster compared with the surrounding corundum. In between
the surface cavities and flux glass are healed areas, indistinguishable
from the surrounding corundum. Photo © R.W. Hughes
Unfortunately, this information never
quite filtered down to the dealer on the street. Small bits of glassy flux residue
were often found on the surfaces of finished stones. Since the amounts involved were
tiny, many assumed the glass to be an accidental by-product of heat treatment. The
reality was far different
– the tiny flux remnants were but droppings on the trail of a massive treatment
beast – one the gem trade has yet to fully confront. It was this secret that
the gem industry was so afraid to bring out in the open.
These are strong words, but carefully chosen. In the late 1990s, the emerald trade
was rocked by non-disclosure of clarity enhancements. Incredibly, what has been done to ruby over the
past decade is far more radical, and yet has completely slipped under the radar.
What the flux is this?
One major factor that separates fine gems from inferior
is clarity. Heavily fractured stones are common in nature, but clean gems are
decidedly not. If you can take a fractured gem and remove the fractures, you
are radically skewing the equation that keeps the prices of fine natural gems
high. What is being done today with Mong Hsu rubies is the removal of fractures.
How often is this treatment applied with respect to Mong Hsu rubies? So often
that over the past decade I can recall seeing only a handful of stones from that deposit which had
open fractures. And yet virtually every piece of Mong Hsu rough is riddled with open fractures prior
Figure 8. Residual flux
Residual flux in a flux-healed fracture within a heat-treated Mong Hsu
ruby. The areas (in the plane of the fracture) between the flux-filled
channels consist of healed ruby. Photo © R.W. Hughes
The flux healing process
Flux healing involves heating corundums with borax
or other fluxes. These fluxes actually dissolve the surfaces, including the internal
surfaces of cracks. The corundum within this molten material then re-deposits
on the fracture surfaces, filling and healing the fractures shut. Undigested
material cools into pockets of flux glass. Essentially this amounts to a microscopic
deposition of synthetic ruby to heal the cracks closed.
In the broadest sense, this is akin to the
oiling of emerald
– both treatments involve reduction of reflections from included cracks/fissures. Similar to
placing an ice cube in water, a filled fracture is much less visible because the filler replaces
air (RI = 1.00) with a substance that has an RI that more closely matches the gem itself (1.76–1.77).
However, the flux healing of Mong Hsu rubies differs in three important respects:
- The Mong Hsu ruby treatment is not a fracture filling,
but a permanent healing of the fractures and fissures, with any filling merely
a remnant of the process. In many respects, it is a welding of fractures, similar
to the joining of two pieces of metal with heat and a flux to lower their melting
- The Mong Hsu ruby treatment is permanent and irreversible.
Unlike the oil in an oiled emerald, flux remnants will not drain out in the future,
nor can they be removed. There is no way to have a stone revert back to the untreated
- The Mong Hsu ruby treatment actually improves a stone’s
durability, since the fractures are permanently healed shut.
Dealing with it
With the explosion of Mong Hsu ruby onto the market,
it became obvious that traditional lab nomenclature was not equipped to deal
with this treatment. Thus in 1997, while directing the colored stone identification
department of the Los Angeles office of European Gem Labs (EGL), I developed
terminology to honestly describe this treatment. The idea was to provide the
customer with an estimate of how this treatment had impacted the gem. A number
of labs (AGTA, GIA, Gübelin, SSEF, GIT, GAAJ) have now adopted elements
of this nomenclature and refined its application (see Lab Manual Harmonization
Committee, 2004). The author’s suggested nomenclature is as follows and
it can (and should) be applied to other treatments:
- Treatment type: Indications
of heating + flux healing of fractures
- Extent: Minor/moderate/significant
number of flux-healed fractures
- Stability: Stable/unstable
under normal wearing conditions
- Prevalence: Never/rarely/commonly/usually/always found
in the market
In the case of glass infilling, the
size of filled cavities is important, with larger fillings having a greater impact
on the appearance and weight of the stone.
The opposite is the case with flux healing. The more perfectly the treatment is
applied, the less residue that might be present in the healed fracture.
We have a superior treatment for Mong Hsu ruby, one
that is actually more stable than ordinary oiling. So why worry?
First, purchasers of ruby are not accustomed to buying heavily fractured stones.
Unlike emerald, clean rubies do exist in nature. Second, the process can also be accomplished with
heat alone. If such stones are deemed acceptable without further comment, what happens when only heat
In the end, the flux-healing treatment should be looked at for what it is, a radical
reconfiguration of the clarity characteristics of a gemstone. If lumped together with simple heat treatment,
it will completely redraw the map not just for ruby, but also potentially for the entire gemstone industry
(think emerald here).
We must stop kidding ourselves. In the eyes of the consumer, high-temperature heating
and flux healing/impregnation of a ruby is not the same as simple cutting and polishing. No amount
of sweet-talk explaining will make it so. A gem that only requires polishing to reveal its beauty is
far more rare than something that needs both polishing and ordinary heating. And that is more rare
than the Mong Hsu ruby, which needs polishing, high-temperature heating and flux-fracture healing to
make it beautiful.
The market should reflect these realities in its descriptions and pricing. Why it
doesn't is because it has not been forced into proper complience. As long as miners, foreign wholesalers,
domestic dealers and jewelers can sell heavily treated rubies as "natural,"
the charade will continue.
Gems and jewelry are luxuries. In the retail market, they compete against a number
of different goods and services. If we don’t make clear distinctions between our different products,
that retail customer may stop buying more than just Mong Hsu rubies.
Postscript: Yehuda-type Pb-glass filled
In March of 2004, the Gemmological Association of
All Japan (GAAJ) encountered a treated ruby in which the fractures were filled
with lead (Pb) glass. This is similar to the Yehuda fracture filling treatment
of diamond. World gem labs have now seen several such stones. Like the glass-filled
diamonds, Pb-glass fracture filled rubies show an iridescent “flash” effect
on filled fractures when examined with oblique illumination under magnification.
A Scientific View of Flux Healing
John L. Emmett
Many gemologists do not clearly distinguish between melting and dissolving. Melting is a definite property of a crystal which occurs at a definite temperature. For example,
if you take a piece of ice at -50°C, and gradually add heat to
the crystal, the temperature increases until it reaches O°C.
Even if you continue to add heat, the crystal stays at 0°C and
begins melting. As more heat is added the temperature remains at
0°C until all of the ice is melted to water, at which point the
temperature will again increase with heat addition. For corundum
this process occurs not at 0°C but at 2045°C. Below this
temperature corundum does not melt.
Table salt melts at 804°C. It
works just likes the ice example above. Below 804°C there is
no melting of salt. However salt can be rendered into a liquid
form by dissolving it in water at temperatures far below its melting
point. Salt dissolves in water at almost any temperature where
water is a liquid, and slightly below.
So now we have two points:
- Crystals have a definite melting point and are
solid below that point and liquid above it. The liquid has the
same composition as the crystal.
- Crystals can become liquefied by dissolving
them in a solvent. This can in principle occur at any temperature.
The liquid formed is a mixture of the crystal (solute) and the solvent liquid, and thus does not have the same composition as the
solid. This can occur at any temperature.
Thus melting and dissolution
are two entirely different processes.
Of solvents and solutes
One frequently reads gemological
articles that make statements like: “the stones
exhibited melting on the corners so the temperature
must have been near the melting point.”
This is incorrect. With
the types of ovens typically used for heat-treating
gems, the crucibles and furnace muffles would break
down before corundum’s 2045°C melting point
In reality, no melting took
place. Dissolution occurred.
The stones were exposed to a material that acted
as a flux (a high temperature solvent) that dissolved
the corners off the stone. This could happen with
some fluxes at temperatures well below 1000°C.
I need to say
more about solvents and solutes. At any given temperature,
a given solvent will dissolve a certain maximum
amount of solute. That is, there will be a fixed
amount of solute per liter (e.g. grams/liter).
Now we need some terminology. If a solution at
a given temperature contains less solute than the
maximum, it is called undersaturated. If it contains exactly the maximum amount, it is called saturated.
If it contains more than the maximum amount, it
is called supersaturated. For most solvent-solute combinations the solubility (grams/liter) increases
with increasing temperature, or decreases with
If a crystal
is put in an undersaturated solution (the solute
is the same as the crystal) it will slowly dissolve.
If it is put in a saturated solution, it will
grow at the same rate as it will dissolve. Thus
the weight will not change. If it is put in a
supersaturated solution, the crystal will slowly
How can a solution become supersaturated?
One simple way is for the solution to become saturated
at one temperature and then be cooled to a lower
temperature. Since the concentration of solute for
saturation is lower at the lower temperature, the
cooled solution is now supersaturated. Supersaturation
is not stable indefinitely; the excess will eventually
solidify by crystallizing out of solution.
Into the void
Now we need to discuss a different
subject, that of the equilibrium shape of a void in
a crystal. First suppose that we have a synthetic corundum
crystal that has a long, thin hot-dog shaped bubble
in it. If we heat that crystal to a high temperature,
the hot dog will break up into a row of spherical bubbles,
and eventually each spherical bubble will become a
negative crystal with facets.
Why does this happen? The
principle is that the shape will change to minimize
the surface free energy.
(Another example of minimizing the free energy of
a system is that given a chance, water will always
flow down hill.) What shape has the minimum free
energy? Spheres are a lower free energy than very
thin tubes. Spheres are also a lower free energy
than a flat crack with a sharp point. In crystals,
negative crystals with low Miller index faces are
lower energy than spheres, etc. This topic is well
covered in Roedder (1984) and references therein.
Figure 9. Necking down
of a void
During healing, as the process continues (left to right),
contraction bubbles form and then become larger as the void
becomes more spherical. The degree of healing in flux-healed
Mong Hsu rubies is so great that the flux remnants are often
quite rounded, complete with contraction bubbles (see Figure 10).
(After Roedder, 1984)
|Figure 10. Flux residue
may be two-phase
Highly magnified photo showing flux residue within a flux-healed
Mong Hsu ruby. Note that the small flux-filled pockets may
contain gas bubbles formed by contraction of the flux residue
as it cools. Photo © R.W. Hughes
How does the material move
to change the shape? In our example of a void in a synthetic crystal,
there are two processes: surface diffusion of
the atoms of the crystal, and evaporation and condensation of the crystal material.
When flux fills a fracture,
it dissolves the wall of the fracture until that small
amount of liquid in the crack becomes saturated with
sapphire solute. As noted above, at that point it dissolves
wall material at the same rate that it deposits fresh
material on the wall. But note that we now have an
efficient mechanism of material transport in the crack.
The system now tries to minimize the surface free energy
of the crack by transporting material from the wall
to the sharp tip of the crack, healing it. Assuming
that we maintain the temperature this process will
continue until the surface free energy differences
from one place in the crack to another are minimal.
Since we are in a crystal the shape will eventually
have crystal facets on the crack (see Figure 2).
This process does a lot of healing, but it cannot completely
fill the crack with new corundum.
Now let’s talk about the
flux itself. Not all the corundum dissolved in it
comes from the wall of the crack. The flux could
have dissolved some corundum from the exterior of
the stones or crucible before entering the crack,
or corundum could have been deliberately added to
the flux prior to treatment. Thus corundum can be
transported into the crack from outside, as well
as being dissolved from the walls.
Assuming some equilibrium configuration has
been achieved in the flux-filled crack, on cooling the flux becomes
supersaturated with corundum and corundum will crystallize out on
the walls of the crack, contributing some additional healing. As
the flux cools further, it solidifies, mostly filling the crack,
but also opening some voids as the flux shrinks as it cools (see
What is the solid flux like, crystal or
glass? Depending on the cooling rate it could be either, or some
of both. Slow cooling favors crystals, while more rapid cooling
favors glasses. If the flux was originally a borate, the solid
material in the crack is mostly an alumino-borate.
Emmett, J.L. (1999) Fluxes and
the heat treatment of ruby and sapphire. Gems & Gemology, Vol. 35,
No. 3, pp. 90–92.
Eppler, W.F. (1959) The origin of healing fissures in
gemstones. Journal of Gemmology, Vol. 7,
No. 2, April, pp. 40–66.
Gemmological Association of All Japan (2004) Lead-glass
Hänni, H.A. (1997–1998) Short notes on some
gemstone treatments. Journal of the Gemmological Association of Hong Kong, Vol. 20, pp. 44–52.
Hänni, Henry A. (2001) Beobachtungen an hitzegehandeltem
Rubin mit künstlicher Rissheilung (Observations on heat-treated ruby with
artificially healed fissures). Zeitschrift der Deutschen Gemmologischen Gesellschaft, Vol. 50,
No. 3, pp. 123–136.
Hughes, R.W. (1984) Surface repaired rubies – a
new gem treatment. Jewellery News Asia, p. 1.
Hughes, R.W., and Galibert, O. (1998) Foreign
affairs: Fracture healing/filling of Mong Hsu ruby. Australian Gemmologist, Vol. 20, No. 2, April–June, pp. 70–74.
Peretti, A. (1993) Foreign substances in Mong Hsu rubies. JewelSiam, Vol. 4, No. 5, p. 42.
Robinson, N.L. (1995) Thais get burned by glass fillings. Colored
Stone, Vol. 8, No. 4, July/August,
p. 1, 6 pp.
Roedder, E. (1962) Ancient fluids in crystals. Scientific
American, Vol. 207, pp. 38–47.
Roedder, E. (1984) Fluid Inclusions. Reviews in Mineralogy, Washington, DC, Mineralogical Society of America, Reviews
in Mineralogy: Vol. 12, 646 pp.
SSEF (1998) Ruby. In Standards & Applications, Basel,
SSEF, pp. 45–60.
The author wishes to thank William Larson, Josh Hall
and John Emmett for fact-checking the manuscript. And a mighty big pat on the back
to Stuart Robertson, who first suggested a revisit of this subject.
About the authors
Richard Hughes is the author of the classic Ruby
& Sapphire and over 100 articles
on various aspects of gemology. He is Webmaster at www.palagems.com and
his writings can be found on his personal web site, www.ruby-sapphire.com.
Dr. John Emmett is one of the world’s
foremost authorities on the heat treatment, physics, chemistry and crystallography
of corundum. He is a former associate director of Lawrence Livermore National
Laboratory and a co-founder of Crystal Chemistry, which is involved with
heat treatment of gemstones.
This article came about via discussions with The
Guide’s Stuart Robertson. Stuart and I had discussed the issue countless
times over several years and he had even attended my lectures on the subject. I
always assumed he and others understood, but one day as I re-explained the process,
something I said flipped the switch and the light went on. Stuart had a "Eureka" moment
and excitedly declared that such knowledge should not be hoarded for personal profit,
but must be made public for the greater good, using illustrations that would illuminate
the subject in a most transparent manner.
Thus the above article was thus born. Penned in July, 2004. Edited versions
appeared in The Guide and IDEX,
 It was later published in the Australian Gemmologist (Hughes & Galibert,
1998), and can be seen at this URL: http://www.ruby-sapphire.com/foreign-affairs.htm.
 Note that this acid is extremely dangerous and should only
be used under controlled laboratory conditions.