Gemstones are enhanced and embellished by many techniques. Any property that a stone may have has a potential for an enhancement. Generally, enhancements are relatively easy to do and cause an appreciable increase in the gemstone's worth. Color and clarity can be enhanced in many gems and there are many methods of doing so. Some enhancements are standard techniques that cause a permanent change, while others are swindles that are done only by disreputable persons.
Though not truly enhancements, fakery gives the unsuspecting or gullible a simulant or composite stone that resembles a natural gems. This topic goes hand and hand with enhancement and is discussed here. Not all composite stones are made to deceive, and for example opal doublets and triplets made with a thin slice of opal really enhance the gem material and even make it easier to fashion and wear. If the consumer is aware of the composite nature of the stone, no harm is done.
Foiling
Adding a shiny backing, such a reflective metal piece is called foiling. The foil reflects light back out of the stone towards the observer. This makes the stone appear more brilliant. A colored foil can also color the stone. What a disappointment when a vintage pink diamond is removed from its mounting on a piece of jewelry and turns into an ordinary colorless stone.
Doublets and Triplets
Composite stones (assembled stones) are assembled from several parts.
This technique is used with cabochoned stones such as opal and jade, and
faceted
gemstones of all kinds.
Opal doublets and triplets are necessitated when thin piece of gem opal that have a play of color are used to create a large piece that is easier to use in jewelry. They also preform a function by giving support that prevents damage to the thin opal piece. A doublet is a piece of opal cemented to a backing of some type. The backing can be ironstone (an opaque clayey stone) or to any other backing that adds rigidity to the stone.
Opal
triplets have both a backing and a top surface that is transparent.
The opal is essentially encapsulated and the top surface is usually, glass,
quartz, or plastic. Obviously, if the top surface is harder than
the opal and resists scratching or chipping (opal itself is very brittle
and shock sensitive) then the triplet is a positive enhancement.
The opal doublet/triplet is done on a cabochon; occasionally fake jade cabochons with a top and backing are filled with a colored substance to enhance the overall appearance of the gem. This is only done as a deception and should be considered a cheat.
Faceted stones can also be made fashioned as composites. The crown may be one substance and the pavilion may be another. An example of this is the diamond doublet. This uses two smaller stones to make a larger gemstone. This increase in size is an enhancement, but is often not reported by the seller and is really a cheat. To avoid being cheated, one should be weary of any plane of bubbles in a stone as the cement between the two parts of the assembled stone will often trap air bubbles when it is fashioned.
The cheat will not put the join between the crown and the pavilion at the girdle since anyone buying a stone is likely to check there. They may join the two parts at an angle to make the plane of bubbles less obvious, and since one purpose of the doublet or triplet is to have a hard upper surface they will usually use a substance of superior hardness of the crown. Jewelers often test the stone’s main facet for hardness or its optical properties and it is best to use either a real piece on the top or at least a substance such as garnet that is harder than glass and will resist scratching.
Triplets may be cemented with a colored substance. For instance,
the Soudé emerald is made using two pieces of glass (also
called paste) with a green cement between them.
Doublets and triplets can be recognized by either direct observation
of their sides, immersion in a liquid makes it easier to observe the components,
or by testing different parts of the stone for hardness or optical variations
that are too different for the stone not to be a composite, or by other
advanced methods such as X-rays. If a composite stone is glued together,
it may fall apart if it is immersed in a solvent such as acetone.
Heating
Heating is the most common method of enhancing as gemstone. For instance, most corundum, ruby and sapphire, is heated to enhance the color and clarity. The heating redistributes elements in the corundum. The heating is very high, close to the melting point of the stone’s. The inclusions such as rutile needles (TiO2) may start to dissolve and be redistributed. Iron may migrate and the color, if patchy, may become both more intense and more evenly distributed in the corundum.
Heating is so universal with corundum and tanzanite that these stones are essentially expected to have undergone the treatment and it is a rarity to find a truly untreated stone of superior color. Unheated stones can be recognized because their inclusions are unaltered. Tanzanite is usually greenish-brown if unheated and so any blue tanzanite stone should be considered as enhanced.
Diffusion Treatment
Diffusion treatment is a relatively recent enhancement technique. It is commonly used of corundum, and it is almost always applied to stones after they have been cut. Diffusion involves heating a stone to near its melting point with the addition of coloring agents. The blue sapphire might be heated with additional titanium and iron. These added substances diffuse, that is work their way into the stone. Usually, they enter only a fraction of a millimeter, but they effectively color the stone. Diffusion is also used to create asterism on corundum creating star rubies and sapphires by adding titanium.
The color enhancement of diffusion treated stones is dramatic, and it is permanent as long as it is not removed by physically polishing or scratching away the stone’s exterior. The stone usually gets pitted by the heating in the diffusion treatment and must be repolished to remove the pits. However, this must be done carefully or the untreated interior of the stone may be reexposed and the effect is diminished.
Since diffusion treatment is permanent, it is an acceptable enhancement, but it must be reported to the consumer under Federal Trade Commission (FTC) guidelines (see Disclosure of Treatments to Gemstones).
Oiling, Impregnation, and Dyeing
Certain gemstones such as Emerald, opal, turquoise, and agates are permeable (will allow fluids to penetrate and flow through them). These stones can accept both dyes and oils and waxes. The filling of the small openings in the stone may allow a new color to be added or allow a wax into the pores that will then make the stone have a glossy finish or fill cracks.
Filling the cracks in emerald reduces reflections and allows better penetration of light into the stone. This enhances the appearance and reduces detrimental effects of cracks that scatter light. Overall the emerald looks better.
Usually a regular oil such as canola oil is used, but resins (both natural and plastics) have been reported. Almost all emeralds sold are oiled. Since it is standard practice, very little is said about it application and most consumers are unaware. Stones need to be reoiled every 5-10 years because of drying. Clearly you should not wash dishes with an emerald on your finger.
Don’t wash dishes with an other porous stone as dirt can enter the stone. Opal is sometimes intentionally darkened by soaking it first in a sugar solution and then burning the sugar using acid (nitric acid, etc.). This “sugaring” is always a deception to make a less valuable off-colored or white opal look like the most valuable type, black opal.
Sugaring is used as a legidimate dye for agates. Agates are cryptocrystalline but are porous and permeable enough to accept dyes. Almost all intensley colored agates are dyed. There are many methods of dyeing stone and agate dyeing was perfected in Germany at Idar-Oberstein.
Turquoise will turn greenish if it absorbs too much oil from the wearer’s skin. Poor quality turquoise can be enhanced by impregnating it with a dyed resin that can turn a chalky textured turquoise into a shiny-bright sky blue.
Fracture Filling
Is the filling of fractures with a substance that has an optical density close to that of the gemstone being enhanced. Lead glass and some plastics or resins are forced into fractures in gems such as ruby and diamond. The process is not much different than what described above, but the increase in value of these gems may be much greater. Often fracture filling is not disclosed and is a cheat. X-rays and careful optical examination may reveal the treatment.
Bleaching
Bleaching is a process that lightens the gemstone. It can be done using caustic substances such as bleach, but also can be done using sunlight. It is most effective for organic gems such as pearls and coral. Gemstones occasionally lose their color if exposed to prolonged sunlight so caution should be taken where a valuable gem is placed.
Irradiation
Though light is a form of irradiation, this technique to enhance gemstones relies on either radioactive substances, particle accelerators, or nuclear reactors. Experiments with diamonds showed that they could be colored by exposure to radium (the radioactive element that was used in the past to make clock hands luminous). These stones turned color, for example creating green diamonds, but also became radioactive themselves. This is a dangerous way to do things.
Recently, the Gemological Institute of America has published information on the effects of using a postal mail sterilizer, a particle accelerator, on gems shipped through the US mail. They found that gems such as pearls, corundum (ruby and sapphire), and quartz changed color, sometimes from just a single pass through the machine!
Radiation from nuclear reactors has been used for changing the color of many gem types. Not all gems turn color permanently. For instance, Maxixe blue beryl (darker than aquamarine) created by irradiation lost its color when exposed to sunlight. Other stones remain permanently changed. In some cases, the gems may become noticeably radioactive and are not really suitable for wearing.
Synthetic Overgrowth
Growing a coating of the right colored mineral on an inferior gemstone of the same composition. This can be done using hydrothermal methods and has be done for emeralds. The thin and colorful overgrowth surrounds a natural nucleus. This is relatively rare and in someways is similar to the diffusion techniques as the outside of the stone is colored in both cases leaving an inferior core.
Laser Drilling
A very thin laser beam is used to drill a hole that extends down to an unsightly inclusion in a gemstone and remove it. The method is mostly used to improve the clarity of a diamond. The clarity refers to both the amount of inclusions and fractures in a diamond. The method often involves bleaching the inclusion (dissolving it away) and may also lend itself to fracture filling of the stone.
Laser drilled diamonds are not as valuable as untreated diamonds, but
the increased clarity can make a dramatic improvement in a rather poor
stone.
Imitations and Simulants
The most imitated gemstone is undoubtably the diamond. There are
many substances that have been used to simulate its appearance. The
list includes: cubic zirconium, Moissanite, Strontium titanate, rutile,
and leaded glass. All of these are common enough that you could run
into them. Cubic zirconium certainly gives the most bang for the
buck (very low prices, good hardness, and good optical properties).
The most recent simulant is moissanite a form of silicon carbide.
It can be recognized by its optical properties, but is close to diamond
in many ways. Moissanite detectors are sold.
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