Due to their rarity and unique visual properties, nearly all colored gemstones are enhanced using various techniques. Many of these techniques have been used for centuries. Colored gemstones that have not been enhanced are very rare and command extravagant prices.
There are many methods of enhancing colored gemstones. The most common enhancements are heat treatment, filling, heating and pressure, impregnation, lasering, infusion, coating, bleaching, dyeing, irradiation, diffusion, waxing/oiling. For example, most rubies and sapphires go through a heating process to improve color and clarity.
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.
These are the methods of enhancing colored gemstones:
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 cabochon stones such as opal and jade, and faceted gemstones of all kinds.
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 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.
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.
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.
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.
Fracture Filling: It’s 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.
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.