Enhancement of gems
Processes which alter the appearance of a gemstone are known in the jewlers' trade as enhancements. While some treatments, such as irradiation, are strictly modern many enhancements of gemstones date back to ancient times. Enhancement is a controversial subject. Pliny condemned the practice and considered it to be a type of fraud. Even among modern jewlers the practice was a taboo subject for discussion, at least where customers were concerned. Modern trade laws, however, require jewlers to reveal all known enhancements.
- cutting & polishing (not strictly an enhancement in the traditional sense, but will be considered here none-the-less)
- dyeing (see also the entry on agate)
- heat treating
- improved heat treating
Modern jewelers do not consider the cutting of gems to be an enhancement since it does not alter the color or texture of the stones. Nevertheless, cutting and polishing are among the oldest means of improving a gemstone's appearance. Traditional cuts include the following:
- baroques (gemstones left in their natural pebble shape are known as baroques. Orginally these were found in river beds—later the technique was developed for polishing baroques in tumbling barrels. Baroque stones preserve the original weight of the gem; also, there was a widespread belief that cutting a gemstone would hurt or mar its mystical powers)
- drilled beads (beaded necklaces were very popular in ancient times, and also among the Norse. The stones would be fashionef as balls or other simple shapes, and then drilled with bow drills. Pliny also mentions that the Indians would leave emeralds and other beryls in their natural crystalline shape—a hexagonal prism—and drill out the pith)
- cabochons (cabochons are gemstones with a rounded dome-top shape; this was a common method for cutting ring stones in the early Middle Ages)
- rough facets
Heating is the oldest known method for enhancing gemstones. Carnelian has been treated by this method for at least 4000 years. The methods used are similiar to those for firing pottery in a kiln. Both ruby and sapphire are routinely heat-treated to improve their color. Aquamarine is also heat-treated to drive out the unwanted green hues. Heat treating can also drastically alter the color of a gemstone. Examples are changing amethyst into citrine, and blue sapphire into white to imitate diamond. Traditional heat-treating was something of a gamble, as there was no way to control temperature or to determine the amount of time needed for the correct color change. Modern heat-treating however is a controlled process, which is considered permanent.
The practice of oiling emeralds to hide surface flaws dates back to ancient times. Typically, cedar oil is used, although modern practice often subtitutes injected resin. The use of green-colored oil to improve the stone's color is considered unethical by most jewlers.