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[Gemlab Report] Emerald Oiling (In-Fill) Process


                   October 1997 - Issue #4

                       by: Ted Themelis

In This Edition:

  • Emerald Oiling (In-Fill) Process

                    Emerald Oiling (In-Fill) Process

Natural gemstone creation, as a rule, is a brutally wrenching
process, often as wounding as it is wondrous. Even so, few
gems take the amount of abuse that emerald does during its
genesis. These emeralds tend to have numerous cracks,
imperfections and birth scars. For this reason, many emeralds
need their looks improved to become presentable and saleable.

Improvement has long consisted of the so-called “oiling
process”, which hides their cracks and fractures, in much the
same way a beauty parlor mudpack hides skin wrinkles.
[Author’s note: in this report, the term “oiling” is
considered as an external description only, since in most
cases, other than true oils are used in the in-fill process].
This camouflage process is so pervasive that it is said that
every emerald destined for jewelry is filled with oil, or with
some other filling substance(s). While that’s an
overstatement, it is hardly an exaggeration to say that this
process is attempted on just about every emerald as a final
step toward market readiness.

And no wonder. All an emerald needs is a surface-breaking
crack that will accept the filler media. With nothing to lose
and everything to gain, it would almost be foolish not to
subject the emeralds to this process. That’s why this
treatment is standard procedure in the world’s main emerald
mining and processing centers: Colombia, Zambia, Brazil,
Israel, India, Hong-Kong, and elsewhere.

There’s just one drawback to emerald “oiling”. The beauty it
brings is nearly always transitory. Every known emerald filler
(oil, oleoresin, epoxy and plastic) will eventually decompose
in time. This means that the constituent elements of the
filler start to separate and that the substance as a whole,
begins to lose its bonding strength with the host emerald.
Chemists call this breakdown “dissociation”.

Once dissociation sets in, the camouflaged emeralds revert to
their original unsightly appearance. Since the loss of their
filler in emeralds is gradual under normal conditions,
consumers may not notice what has happened, or ignore it when
it occurs - especially when they have not been told that their
emeralds were subjected to “oiling”. Granted, disclosure is a
rather delicate duty with a treatment as impermanent as
"oiling". Yet, it is a duty, which must be done, especially
since the in-fill media loss can be speeded up from the
gradual to instantaneous when uninformed owners bath emeralds
in ultrasonic cleaners.

The pity of non-disclosure is that most dried-out emeralds can
be restored to their beauty by “re-oiling” them. Members of
the jewelry industry must take a positive approach to
disclosure - first explaining the benefits of “oiling”, then
encouraging customers to bring in their emeralds for periodic
inspection and, if needed, reconditioning.

To be blunt, most “oiled” emeralds sooner or later would
require a lube job. Nevertheless, revealing this fact of life
doesn’t have to be an unpleasant chore. Instead, disclosure
can be used as a potent sales tool to keep customers coming
back, if only to have their emeralds checked.

This is where an understanding and appreciation of “oiling” is
important. Contrary to the popular impression, successful
"oiling" is a sophisticated process that cannot be done simply
by plunking dried-out emeralds in trays of, say Mazola oil.
Rather, this treatment should be left to professionals versed
in its intricacies. This means that the emeralds must be sent
to special labs where residual oil (or other fillers) can be
removed and new filler applied.

Consumers and members of the gems/jewelry industry should
realize that treatment is a combination of art and science;
the processing techniques vary from lab to lab, ranging from
primitive to highly sophisticated. Nevertheless, this report
covers the current mainstream and avant-garde techniques of
emerald “oiling”, as practiced by the author Ted Themelis, at
his laboratory. It divulges some secrets of the treater’s art
known up to this point only by a few trade insiders, in the
hope that demystifying the subject of “oiling” will promote
disclosure of this increasingly pervasive and essential trade

Ends and Means
Emeralds are “oiled” to reduce the visibility of cracks and
internal imperfections, thus improving their overall
appearance. Flawed or poor-quality emeralds which have cracks,
crevices or fissures that break the surface are candidates for
this process, since these openings provide the only entry
point for the filling media. Although some treaters restrict
themselves to one or two filler substances, most high-tech
treaters choose from a wide selection of fillers, depending on
the size, configuration of the cracks, their knowledge,
available equipment and other parameters. The success of the
"oiling" process in emerald, depends on the ability of the
chosen filler to be induced into the surface-breaking cracks
of the emerald and remain there, as long as possible. Rough,
pre-formed, faceted and cabochon emeralds are suitable for the
"oiling" process, as long as these emeralds have crack(s)
reaching the surface.

Fillers (In-Fill Media)
Basic fillers

 Today, emeralds are enhanced using three types of filler
 substances: Organic and synthetic oils (usually Cedarwood
 oil) Oleoresins (most commonly Canada balsam) Synthetic
 epoxy resins (nearly always Opticon). 
  1. Oils (organic and synthetic). In the past, just about every
    conceivable organic oil (including castor, coconut,
    cottonwood, mineral, palm, peanut, olive, whale’s oil and
    other oils) or a combination of these oils, has been used as
    filler in emeralds. Unfortunately, most of these oils have two
    strikes against them discourage use by professional treaters.
    First, their refractive indices are under 1.50; second, their
    viscosity is too low. While the former has some bearing on the
    treatment’s effectiveness, the latter influences both its
    stability and longevity. If viscosity is too low, the filler
    will tend to be thin and loose, likely to dry-out quickly and
    exit from the emerald’s substance. Treaters prefer
    high-viscosity fluid fillers that, at room temperature, are
    thick, syrupy and sticky in their natural state, but which
    lend them to thinning during induction into the emerald’s
    substance. Of the scores of natural organic oils that can used
    as emerald fillers, Cedarwood oil has merged as favorite,
    although the quality and physical properties vary, as received
    from the suppliers.

Cedarwood oil is transparent, colorless to slightly yellow,
volatile (readily vaporized), somewhat viscous, insoluble in
water, but soluble in ether and pure alcohol (which make the
latter two ideal solvents to clean out Cedarwood oil residues
before re-oiling), sensitive to other than normal room
temperature and prone to chemical alteration from light
exposure. Cedarwood oil has a refractive index of 1.495-1.510
and its specific gravity is about 0.94-0.95 gr/cm3.

  1. Oleoresins. Canada balsam is another favorite filler among
    treaters. Canada balsam is not exactly an oil, but rather an
    oleoresin - a solution of resin in essential oil extracted
    from the North American balsam fir tree. Canada balsam is
    transparent, yellowish to slightly greenish, viscous, slightly
    fluorescent, sensitive to both light and temperature,
    insoluble in water, completely soluble in ether or oil
    turpentine and only 90% so in alcohol (making all three good
    cleaning agents for stones filled with this oleoresin).
    Specific gravity is about 0.980-0.994 gr/cm3 and its
    refractive index ranges from 1.52 to 1.54.

  2. Epoxy/resin fillers. In recent years, epoxy resins
    (polymers used as adhesives) have begun to vie with
    traditional oils and oleoresins as standard in-fill mediums.
    The most popular of them is synthetic polyester epoxy resin,
    marketed by the trade-name “Opticon”, famous as a fracture
    sealer that has endeared itself to treaters because is more
    stable than oil. Opticon is transparent, light amber in color,
    slightly fluorescent and comparable in viscosity to that of
    Canada balsam. It has refractive index of about 1.545.

Despite its virtues, Opticon is still an impermanent emerald
filler. Treaters can extend its life by adding in its
companion hardener, at one part to every 10 parts of Opticon
or any other desirable ratio. Few do so, however, because
removal of hardened Opticon is very difficult should it become
necessary as a result of chemical breakdown of the filler.
Ethyl-based ketones have been used as cleaning agents, in
chemical condenser apparatus, but only with limited success.

Various agents, such as plasticizers and hardeners, have been
added to these fillers which contribute to their penetration
and life expectancy.

Because oil and epoxy resin fillers prove unstable over time
and exit partially from the emeralds, treaters are
experimenting with additives known as plasticizers that act
both as filler-stabilizers and sealants. These additives mixed
with fillers, increase their viscosity and resistance to
breakdown. When they are applied as a surface coating over the
filler, they form a closure that reduces greatly and in
sometimes stops seepage.

Although plasticizers extend the life of impregnation
treatment, they don’t make them permanent. In time, these
stabilizers/sealants lose their initial strength and dry-out,
creating dust-like whitish particles within voids. Painstaking
cleaning is then required, before the emeralds can be

Multitudes of pale low-grade beryl can be converted to emerald
by adding green organic dyes to conventional filler
substances. In general, “greening” emerald by means of
artificial coloring is condemned by many gem dealers and
consumers as unethical, although widely employed in India,
Brazil and elsewhere. Defenders of this practice argue that it
is arbitrary to approve of “oiling”, but not of dyeing when
each serves the same purpose of improved looks. Yet there is a
rationale for censuring this treatment. Organic dyes speed up
the breakdown of the fillers to which they’ve been added
jeopardizing the beauty they help create.

Chosing fillers
Although the range of these in-fill media vary widely,
treaters narrow their choices according to certain criteria,
such as: 1) Refractive index (which should be as close to
that of emerald as possible) 2) Viscosity (flow
characteristics) 3) Solubility 4) Fluorescence 5)
Environmental sensitivity (light and temperature).

In choosing fillers, treaters determine the following:

  1. Single or multiple fillers (type and ratio)
  2. Hardener to be used in the in-fill process
  3. Dye to be used in the in-fill process

Cedarwood oil and Canada balsam are sometimes mixed at
approximately a 3:1 ratio for use as emerald in-fill. What’s
more, Canada balsam has been mixed with other oils as an
in-fill for other gems. An untold number of mixing
combinations and ratios involving oils, oleoresins, epoxies
have been applied to produce the final filling media. The
results after the treatment vary significantly and depend on
many parameters.

Identification of the Filled Emeralds
Generally, an experienced gemologist can identify whether or
not the emerald in question is "oiled’. This is achieved by
observing its inclusions residing at or near the cracks,
crevices and fractures, using any microscope equipped with a
dark-field illumination and a fiber optic unit. Some dry-out
filler media in emerald’s cavities stand prominent as minute,
white dots, or specks, having characteristic appearance and
configuration. Other fillers appear as “rusty”, “burnt”,
especially when heating was incorporated during the "oiling"

Immersion microscopy techniques may be used to identify
"oiled" emeralds. The emerald to be examined is embedded in a
glass transparent crucible with flat walls containing an
immersion liquid whose refractive index is very close to that
of the emerald. A suitable liquid would be bromoform whose
nD=1.56 is very close to that of emerald, which is around
1.57. Once an emerald is observed embedded in bromoform, all
that will be visible are those areas that contain lower
refractive index filler. As for the emerald whose fillers have
green dyes with lower refractive indices mixed into them,
these dyes will give themselves away as small, localized
patches of color confined to the small areas of cracks into
which the dye was forced.

Fluorescence is also used to determine the fillers in the
emerald substance. Some treaters mix purposely the selected
filler with fluorescent dyes, such as “fluoresin”, to aid the

In many cases, the use of the hot-point instrument is very
helpful. The hot-point is applied at the emerald’s surface
cracks; the thermal reaction causes the oil to "bead-up"
producing characteristic inclusions.

When one of more fillers were mixed together with or without
added hardener to compose the final filler media, the precise
determination of these filler ingredients residing in the
emerald’s substances is extremely difficult and cannot be
achieved using conventional gemological instruments.
Furthermore, it is impossible to determine when the filler
media was induced into the emerald’s substance. The
determination of these fillers, may be achieved with using
various spectrophotometer techniques, employing sophisticated
instruments, distant to average gemologists.

The “Oiling” Process
Before the typical crack-infested emerald is “oiled”, it looks
a lot like an ice-skating rink in need of surfacing. Stones
show feathers running in every direction and configuration.
The goal of “oiling” is to conceal or minimize all of the
emerald’s surface-breaking fissures and also hide inclusions
trapped within them.

Basically, “oiling” is a three-step process involving 1)
removal of air and gases from the cavities of the emerald 2)
application and induction of the filler using high-pressure
and 3) final cleaning and waxing. Emerald oiling experts are
well aware that among treatable emeralds some are far more
enhancement-friendly than others. Acceptance of the filler
substances depends on the dimensions of permeable openings.
What’s more, residues from decomposed or incomplete previously
treatments frequently obstruct penetration of new filler.
Furthermore, if stabilizers are used, “oiling” becomes a
five-step process that begins with a thorough cleaning of the

Cleaning. This step is necessary only for previously "oiled"
stones. No matter what the filler, it almost always leaves
sediments behind when it decomposes. So, before previously
filled emeralds can be re-treated, they must be thoroughly
cleaned. Removal of plain oil residues is a relatively easy
task; cleaning emeralds of leftover oleoresins and epoxy
resins are much harder jobs. But the hardest cleaning job, by
far, is the elimination of filler remnants to which hardening
agents (stabilizers and plasticizers) were added.

Cleaning emeralds involves presoaking them in the appropriate
solvent solution. Heating the solution to a low temperature,
say 150 to 250 F, helps the solvent interact better with the
remaining filler and quickens cleaning.

Selection of the solvent is determined by the nature of the
filler being removed. The author has found best results using
the chemical condenser incorporated with automatic control
heating apparatus, utilizing ketone-based cleaning agents,
methylene chloride, super-saturated xylene and lacquer
thinner. Because a single immersion in a solvent usually does
not rid stones of all foreign matter, the cleaning process
must often be repeated several times. The entire process can
take from hours to days. There is no specific rule, method or
pattern to follow.

  1. Removal of air process. Once a previously “oiled” emerald
    is cleaned of lingering filler, it is placed in a vacuum
    chamber where air and gases are removed. It is important that
    no air remain in fissures meant to receive filler because the
    presence of air will create bubbles in the substance. Only
    powerful and efficient vacuum pumps will ensure air-and
    gas-free fissures.

  2. Filler application process. Immediately following the
    suction of air and gases from cracks, the emerald is embedded
    into the filler substance for induction. Pressures of up to a
    3,000 pounds per square inch are used. To reduce its viscosity
    (resistance to flow) and boost its penetration power, the
    filler is thinned by heating to anywhere from 150 to 250 F,
    while is pressurized. After being forced into the emerald, the
    filler is allowed to cool and its viscosity level rise high
    enough to prevent seepage. Although the filler process follows
    the same outline in most labs, the actual procedures may be
    performed slightly differently, depending on the treater’s
    skill and technical know-how. For instance, in some cases the
    author uses a radio-frequency thermal wave transmitter of his
    own design to heat the filler and reduce its viscosity during
    induction. A custom-made hydraulic pressure apparatus ensures
    the maximum impregnation of the filler into the emerald (see
    photo of emerald enhanced apparatus, appearing in my web

  3. Stabilization process. In some cases and under suitable
    conditions, various stabilizers are applied in emerald. The
    selected “add-on filler-plasticizer” is actually a cured
    sealant that is usually applied after the filler is induced
    into the emerald substance and under suitable conditions.

  4. Application of wax process. After induction of the filler,
    the emerald is cleaned thoroughly, externally. Either wax or
    Vaseline is applied to its surface with a chamois cloth to
    give the stone its final finish and luster.

Proper handling
Since the majority of emeralds are “oiled”, caution should be
exercised when they are handled loose or mounted. Stone
setters should be instructed to assume emeralds are all
"oiled" - unless told otherwise - so as not to damage these
emeralds whose cracks have been successfully concealed. Bench
men should also be warned that undue heat applied to "oiled"
emeralds during jewelry repair or fabrication can char stones,
causing serious and irreversible damage.

Cleaning treated emeralds even with simple solvents such as
benzene or ether-based compounds is to be avoided, especially
in an ultrasonic cleaner. This process may remove the filler
and leave stones in their original unsightly condition.

When storing treated emeralds in stone papers, do not wrap
them in cotton. Cotton absorbs oil (as well as dye) over a
period of time. Also, keeping oiled emeralds in hermetically
sealed plastic bags is not recommended; these tend to build up
moisture that weakens and draws out the oil.

Since most of fillers are light-sensitive, it is best not to
display “oiled” emeralds under flood light or any other strong
heat-producing lighting conditions, such as tungsten lamps.
Strong lights produce excessive heat that dry out the oil very
quickly and in time turn the emeralds dull and opaquish.

All rights reserved. Copyright Ted Themelis Users are
permitted to download the for their own private,
non-commercial use. Any other use or reproduction of this
document (text or graphics) without the express written
consent of Ted Themelis is strictly

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