Recognising the difference between treated, synthetic and imitation gemstones is one of the greatest challenges of Gemmology. Often in addition to microscopy, physical and chemical analyses are necessary, which requires a well-equipped gemmological laboratory.
Gemstones are treated with different methods in order to improve their appearance in colour and or clarity. Heating, irradiation, cracking, impregnation and dyeing are the most frequently used methods. Most of these methods diminish the value of the stone and must be unambiguously declared as such.
Equally mandatory is the statement of a synthetic or imitation gemstone. A synthetic gemstone is an artificially produced gemstone with the same basic chemical composition, growth structure and physical propertied as its natural counterpart. An imitation, on the other hand, imitates a gemstone by using a different, deceptively similar material.
Treatments, synthetics and imitations must be clearly declared in the trade, as well as to the final consumer. International trade rules describe exactly how these should be declared (Please refer to the World Jewellery Federation CIBJO). Another reference concerning this matter is the UBOS Booklet;
From the beauty of the precious stones and their treatments.
The brilliance of the diamond in a lower grade can be reduced by dark cracks or inclusions. Cracks can be filled by a highly-refractive, glassy substance, making the cracks less visible and thus giving the impression of a higher purity. Dark inclusions can be targeted by laser, dissolved and extracted. The channel of the laser beam remains visible and so is often filled with a vitreous substance.
The colour of diamonds can be significantly improved by subjecting the diamond to radioactive irradiation and subsequent heating. More recently, certain types of diamonds have been subjected to treatment of extremely high pressures and temperatures ("HTHP treatment") to suppress the effect of an undesirable colour shade.
Rubies and Sapphire
Since the beginning of time, rubies have been heated to obtain a more uniform or desirable red. Rubies of lesser quality, exhibiting fine cracks and small notches, can also be filled with a vitreous substance during heating in order to improve their apparent clarity.
Like rubies, sapphires are frequently heated to improve the intensity of the blue and to improve the purity by dissolving fine inclusions. For some time now, diffusion treatment has also been applied, in which sapphires are heated almost to melting point in a colouring corundum powder, resulting in a thin blue colour layer on the surface of the stone.
In nature, emeralds often contain inclusions called "garden" which diminishes the stone’s clarity. For as long as we can remember, these fissures have been filled with colourless oils in order to make them less visible to the naked eye. This treatment is reversible and therefore less serious. However, sometimes coloured oils and colourless synthetic resins, are used and these treatments are far more serious as they are deceptive and dangerous in the case of resins as these can harden in the stone and in the worst case, can damage the stone.
Aquamarine, Zircon, Topaz, Amethyst, Tanzanite
Aquamarines, zircons, amethysts and tanzanites are very frequently heated to change or intensify their colour: Greenish blue aquamarines become light blue; Yellow and brown zircons become colourless or blue-green; Violet amethysts become yellow citrine; Brown-green tanzanites become blue-violet. Colourless topazes become brown by radioactive irradiation and then blue after heating.
Turquoise, Lapis Lazuli, Chalcedony, Jade
By colouring and impregnation, these porous or cryptocrystalline gemstones can be improved in colour and durability.
Cultured pearls are generally polished to enhance their lustre, slightly spotted cultured pearls are bleached, while white cultured pearls can be coloured grey, yellow and brown. Their colour can also be permanently changed by irradiation.