Emerald dealers are faced with a real dilemma. They sell a stone that, in its natural state, normally has many eye-visible cracks and other flaws. Their customers, however, want stones that look clean to the naked eye.
If the fractures are filled with an appropriate substance, they’re less noticeable and the overall colour may improve. Consequently, fracture filling has become a standard treatment for emeralds.
For this reasons Emerald is a gemstone which the collector must approach with fear and trembling. A majority of the emerald available on the world market is treated with a variety of substances to enhance the colour and clarity of the stone. Rough emerald is often highly fractured. Since earliest times, various oils have been used to hide cracks to improve the clarity of the gem. This practice has’ been going on for so long that it has become accepted and is rarely disclosed to the buyer. In recent years», polymer plastics have begun to replace the traditional oiling.
Opticon is the brand name of a popular polymer with a refractive index that virtually matches the refractive index of emerald, making it impossible to detect without sophisticated testing. ln addition, green dye may be introduced into the polymer filling, which can improve the apparent colour of the stone dramatically.
Current industry opinion is that oiling, and to a lesser extent the use of unhardened polymers, is acceptable, though dyeing is not. However, in practice, many dealers turn a blind eye to both types of enhancement.
Given the high prices of fine emerald, collectors are advised to insist on full written disclosure backed up by an expert laboratory analysis before finalizing the purchase of an emerald. Recently many labs around the world have adopted a uniform seven-step classification to describe emeralds that have been» treated with colourless oils or polymers. The classifications are as follows: none, no significant, faint, faint to moderate, moderate to strong, strong to prominent, and prominent. Stones that fall into the first four classifications, none to faint to moderate, are rare and worthy of consideration. Regardless of appearance, stones that fall into the last two classifications, strong and strong to prominent, are best avoided. The aficionado should bear in mind that these are grade levels of treatment, not clarity. That is to say, an emerald graded none is not necessarily a flawless stone, it is simply a stone that has not been oiled or treated with polymers. An untreated emerald can still look like a piece of a broken Coke bottle. A stone may appear flawless to the eye and still be found to have a prominent level of treatment. This means that the treatment has effectively covered up many sins.
Emerald filling substances:
Essential Oils. An example of this type is cedarwood oil, which is one of the most widely accepted emerald fillers. Essential oils are normally liquid at room temperature, but some are semisolid and several are solid. When exposed to air, they’re not stable over time. However, if oils discolour, they can be easily removed and replaced. In other words, the treatment process is reversible. Some dealers say it’s easier to assess the inherent quality of oiled emeralds than those treated with some other substances. They claim the oil just masks the flaws; it doesn’t hide them.
Other Oils. Mineral and vegetable oils are classified as other oils because they have a different viscosity, volatility and chemistry than essential oils.
Natural Resins. Canada balsam, another well-accepted emerald filler, is generally considered a natural resin. It’s usually more durable than oil. Artificial Resins (Epoxy & other prepolymers and polymers). The epoxy resin Opticon, a filler that has been used in Brazilian emeralds, is an example of an artificial resin. Fillers can be divided into four groups-soft (liquid) fillers, semi hard fillers, surface-hardened fillers, and hard fillers (classification from the Summer 2007 issue of Gems & Gemmology, p 122). Resins can discolour. When the discolouration and filler cannot be removed, the filler is not well accepted.
Some of the newer man-made resins classified as polymers have gained acceptance because they’re more stable than fillers used in the past and don’t turn white. One example is the filler named ExCel.
Wax fillers have relatively good stability and are often used in Zambian emeralds. Common waxes include beeswax, vegetable wax and mineral waxes. Colourless and near colourless filling substances often enhance the colour of an emerald by making it appear more transparent and less milky. Occasionally, green dye is added to a filler to further improve on the colour. Thus, you should suspect a coloured filler whenever you see bargain-priced emeralds with an intense green colour. Most trade members consider the use of coloured fillings an unacceptable practice, especially if it’s not disclosed.
Detecting and Identifying Emerald Fillings
Emeralds must have fractures in order to be fracture-filled, and the fractures must reach the surface of the stone at some point. Otherwise, a filling cannot be introduced into the stone. Therefore, to detect fillings in emeralds, you should look for breaks on their surface. Sometimes these are visible with the naked eye.
But usually magnification is needed. When light is reflected onto the stone, the surface fractures are easier to spot.
If an emerald has surface cracks, it has probably been clarity enhanced (fracture-filled to improve its clarity). If the cracks are numerous or large, the change in clarity could be significant. Keep in mind that fillers are used to either de-emphasize or hide fractures. Consequently, filled fractures are sometimes very
hard to locate. The stone should be viewed through a microscope from many different angles using darkfield illumination and reflected light. Lay people need professional assistance.
Gemmologists and dealers use a variety of clues to identify emerald fillings and determine their impact on the clarity. Some are:
Orange or yellow colour flashes in the fractures as the stone is rocked back and forth. In some emeralds, the orange alternates to a blue flash. This orange/yellow and yellow/orange-to-blue flash effect is commonly seen in stones treated with Opticon and other epoxies, especially if a hardener has been added. It’s not a characteristic of oiled stones.
You can see the colour flashes in emeralds most easily under a microscope, but they’re often visible through a 10-power hand magnifier. Keep in mind that the colour flashes must be along the fractures. Non treated stones will show blue and orange/yellow flashes off the facets but not along the cracks.
Epoxy-treated stones don’t always display orange/yellow or blue colour flashes along breaks. Oil fillings occasionally display a multicoloured iridescence
Flash effect on oil treated emerald
Residue and air bubbles These can be found in oil-and epoxy-filled fractures. Inexperienced viewers might mistake natural emerald inclusions for filling residue and vice versa.
Evidence of treatment on emerald
Ultraviolet fluorescence. Many oil fillings show a yellowish-green to greenish-yellow fluorescence under long-wave ultraviolet light. Other fillers may be bluish or inert. Keep in mind that fluorescence is merely an indication. Some stones with oil don’t fluoresce. Stones filled with both oil and epoxy resin can display the same fluorescence as those treated only with oil.
Thermal reaction. When a hot needle is put next to a fracture opening, oil will “sweat” and form beads along the edge of the fracture. An epoxy resin sealed at the surface with a hardener does not do this. However, movement of the epoxy in the fractures may be visible under magnification. This thermal test should only be performed by professionals because if it’s not done properly, the hot needle can cause the stone to crack or shatter.
Reaction to immersion in solvents. Some dealers place emeralds in acetone or alcohol to determine the extent and type of filling. Most oils dissolve in acetone whereas hardened epoxy doesn’t. Evidence of filler removal can be seen under magnification. Stronger solvents such as methylene chloride are used to dissolve epoxy fillings.
Some jewellers spot-check their emerald jewellery for coloured oils by placing it in a solvent. If the colour of the emerald(s) becomes a lot lighter, the jewellers know a coloured filler has been used, and they return the merchandise.
Lay people and appraisers should not use solvents to test an emerald. The emerald will probably look less attractive if the filling is dissolved. Afterwards, they’ll have to pay to get it professionally reoiled. For best results, emerald oiling should be done with special oils and sophisticated vacuum processes. Simple immersion in the proper oils sometimes works, but it doesn’t always restore the emerald to its original filled state.
Tips on Buying Treated Emeralds Eye-clean untreated emeralds are available, but they’re rare and they command premium prices. Emeralds typically have more fractures than other gemstones because they form in geologic environments characterized by more abrupt changes and mechanical stress. Fracture filling helps mask the fractures, making the emeralds more attractive.
If you’re buying a $75 half-carat emerald, the only treatment information you need to know is that the emerald has probably been fracture filled with a colourless filler and that it therefore requires special care. For example, the emerald should not be steam cleaned or placed in an ultrasonic cleaner or in solvents such as alcohol and acetone. Instead you should clean emeralds by simply wiping them with a warm soapy cloth, rinsing them in water and drying them with a lint-free cloth. Coloured fillers should be disclosed to buyers, even when the price is low.
As the price of an emerald increases, the more you need to know about its clarity enhancement. Before buying expensive emeralds, it’s helpful to have information about the:
- Extent of the treatment
- Type of treatment
- Effect of fracture filling on price