There are various techniques of modifications intended to improve the appearance of precious stones, namely:
Traditional treatments: the heat treatment of the corundums (rubies and sapphires) and colorless oiling of emeralds are practised since several hundred years. They are « tolerated » treatments.
Recent treatments, known as specific, by diffusion, irridiation, bombardment, colored oiling, filling by resins, high pressure/high temperature, result from the new techniques of development. They must be obligatorily mentioned at the customer.
Heat treatment will improve the depth of color because heat causes dissolution of inclusions and diffusion of impurites (especially Ti from rutile inclusions) into the surrounding corundum. Because fine inclusions cause some stones to look cloudy, heat treatment that dissolves the inclusions may also improve the clarity of the stone.
Heat treatment may also remove local color concentrations (remove patches of color) because heat allows the color-causing impurity (Cr, in the case of ruby) to more evenly distribute through the crystal. (however, this may require such long times that it is impractical).
Diffusion is a means of changing the color of a gemstone by heating under pressure in a particular chemical environment (generally, a gas enriched in a particular element). Historically, this produced a color change confined to a shallow region near the surface of the stone. Consequently, chips or scratches could sometimes expose the original color of the stone.
Nearly all natural emeralds have extensive internal flaws. The appearance of these stones is markedly enhanced by first, placing them in a vacuum chamber and then immersing them in cedarwood oil (or another colorless oil). When the vacuum is removed, the oil is pulled into the minute spaces between fracture planes, making them less visually apparent.
This treatment can produce a remarkable change in the appearance of an emerald, greatly enhancing the beauty and clarity of the stone.
In recent years, several alternative filling materials have come to market. The most famous is « Opticon ». Opticon and other epoxy resins are used in conjunction with hardeners.
Clarity enhanced: diamonds are occasionally laser drilled to eliminate dark inclusions. Occasionally some diamonds are fracture filled with a foreign substance, which has good stability except during repair. Fracture filled is the process by which a substance of the same optical density as that of a gemstone is pressure filled into a fracture of that gemstone. These diamonds are called Clarity Enhanced and must be disclosed.
Irridiation: irradiation of diamonds has been around for a long time and uses a nuclear reactor or linear accelerator to create fancy colored stones. These are generally over colored and then annealed or heated to soften the shade back to a desirable and permanent color. Such treatments need to be disclosed by merchants/vendors as well as labs, it is illegal not to.
HPHT process: this process, using a high pressure/high temperature process (HPHT), changes inexpensively brown diamonds into near colorless diamonds worth more than twice as much.
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