Gemstones are graded according to their 4Cs, just like diamonds.
Color is a predominant factor when choosing a gemstone. Gemstone color is defined in terms of hue, saturation and tone. Hue is the primary gemstone color, saturation is the purity or intensity of color and tone is the lightness or darkness of color. GIA defines 31 gemstone hues, six levels of color saturation and a 0-10 scale for tone. Best and most valuable gemstones are those that exhibit only one pure hue with slight traces of additional hues.
Similarly to diamonds, gemstones also have inclusions and blemishes. The mere existence of inclusions and blemishes is not so important as their position and consequently their visibility. The least visible inclusions are those under the bezel facets and near the girdle. Gemstones are usually divided into three categories. The first category comprises gemstones that are generally inclusion-free, such as Amethyst, Citrine, Topaz and Zircon. The gemstones from the second category usually have a few inclusions and blemishes. Some of them are Alexandrite, Ruby, Sapphire, Spinel and Tourmaline. The third category contains gemstones that are almost always included, such as Emerald, Red Tourmaline and Red Beryl. Gemstone clarity grades range from Very Very Slightly Included (VVS) to Included 3 (I3). VVS grade stands for gemstones without any inclusions or with inclusions so minute that are hard to spot under 10x magnification. Included 3 grade describes highly prominent inclusions that have very negative effect on gemstone clarity and consequently its price.
Gemstones are typically cut to highlight their color, although carat weight, brilliance, symmetry and polish are taken into consideration as well. Depending on the cut, gemstones may be faceted or cabochon cut. Transparent gemstones are faceted to maximize their light exposure and brilliance, whereas opaque gemstones are cut as cabochons to emphasize their color. Gemstones with highly saturated color may have shallower cut, whereas those with low color saturation allow deeper cut. Good gemstones are well proportioned and symmetrically cut.
The greater the carat weight, the higher the price of a gemstone, provided that its cut, clarity and color are satisfying as well. However, the carat weight is not the same as the size of a gemstone, due to the fact that the specific gravity or density is not the same for all gemstones. For example, 1 carat sapphire is smaller than 1 carat diamond, as a sapphire has greater specific gravity than a diamond.
Gemstones undergo various treatments to enhance their appearance and accentuate their beauty. Almost all gemstones are treated in some way; those untreated are very rare and expensive. Some of the treatments, like heat treatment, have been in use for centuries so far and are now considered a standard procedure. Any gemstone treatment should be clearly disclosed, as some of the treatments are hard to spot or may improve a gemstone’s appearance but compromise its durability.
Heat is applied to gemstones to permanently change or enhance their color and clarity. This treatment is stable, oldest and most commonly used of all, widely accepted by the jewelry industry. Gemstones that are most frequently heat-treated are sapphires, rubies, tourmaline, zircon, topaz, citrine, amber, aquamarine and amethyst. Heat treatment is irreversible and usually not discernible. Although heat treatment generally does not affect gemstone durability, certain measures of precaution are advised as heat may render some gemstones brittle.
Fracture Filling and Flux Healing
These are also among the oldest and commonly used gemstone enhancement techniques practiced for centuries. Such treatments make a gemstone’s natural inclusions less visible and improve its clarity. Fracture filling implies filling a gemstone’s cracks, fissures and cavities with colored or colorless substances like wax, oil, glass, plastic or resin. Flux healing is usually applied in combination with heat and implies infusion of gemstones with the molten flux of chemicals that will melt the inclusions and close fractures that were once open. As emeralds almost always have considerable number of natural inclusions, fracture filling improves their appearance. Rubies are also frequently filled with glass to make them more transparent or to make their surface reaching cavities less visible. Gemstones enhanced in this way must be carefully cut to avoid the leakage of infused substances. Exposures to high temperatures or to changes in air pressure are also to be avoided as this may alter the substances used as fillers.
Irradiation involves gemstones being exposed to radiation in order to permanently alter and enhance their color. A gemstone is bombarded by electrons or neutrons in a nuclear reactor to change the atomic structure of its crystal lattice and consequently change a gemstone’s light interaction, which results in its color change. Irradiation is usually followed by heat treatment. Blue topaz is usually color enhanced this way.
This treatment serves as the surface enhancement and involves gemstones being lacquered, enameled, foiled or inked to improve their durability and appearance by hiding the blemishes or intensifying the color. Although this treatment is not considered very stable or permanent as the coating may be prone to scratching, it is frequently used, especially with topaz, tanzanite, cubic zirconia and quartz.
Gemstones are infused with dye to change or enhance their natural color and to make the color more uniform. Pearls and black onyx, famous for its rich color, are dyed this way. This treatment may not be permanent if the dye used is unstable and of low quality, easily removed by chemicals such as ammonia, acetone or alcohol.
This treatment is hard to discern and it involves the use of chemicals, light or heat to lighten the gemstone color, remove its color or improve color uniformity. Some gemstones are first bleached and then dyed. Jadeite jade and pearls are often bleached, usually with hydrogen peroxide. Bleaching may make gemstones more prone to breakage, thus, bleaching is frequently followed by impregnation to improve gemstone durability.
Gemstones usually fall into two categories,
precious and semi-precious, although the American Gem Trade Association (AGTA) deems such division deceptive and disapproves of the term semi-precious.
Diamonds, sapphires, emeralds and rubies are considered precious gemstones due to their alluring beauty, rarity and hardness. Other gemstones are not so hard and can be more frequently found, thus the label semi-precious. Another not so precise division is between diamonds and colored stones, as there are fancy color diamonds and a colorless sapphire that is regarded as a colored stone.
Diamonds are considered to be the most amazing and unique of all gemstones, as they are the only gemstones formed out of just one element (carbon) and their journey from simple graphite to brilliant diamonds is long and strenuous. The hardest stone on the Mohs scale of mineral hardness, diamonds are extremely rare and thus quite pricy and attention grabbing. Their price is even higher when they come in various colors, ranging from yellow to black.