Just as with diamonds, the 4 C’s also determine the quality of colored gemstones. Cut and Carat weight are the same as for diamonds. However, Color and Clarity are judged differently.
Many experts consider the beautiful color of a gemstone to be not only its most defining characteristic, but to be the most important evaluation criterion.
When describing a gemstone’s color, experts will examine and describe the gem’s hue, tone, and color saturation. Hue refers to other colors that are in addition to the primary color of the stone.
Tone is a way to describe the depth of color present. Tone is generally described in terms of “light”, “medium-light”, “medium”, “medium-dark” and “dark”. Saturation, also known as color purity, refers to the amount of brown or gray hues that are present.
Colored gemstones that have only minor amounts of gray or brown hues are frequently described as having a “strong” or “vivid” color saturation.
The GIA has developed a clarity grading scale for colored gemstones. They are broken down into three types or classes. Type 1 gemstones are those that are normally found to be clean and include: Amethyst, Aquamarine, Blue Topaz, Citrine, Kunzite, Tanzanite, yellow Beryl and yellow Chrysoberyl. Type 2 gemstones normally have a few inclusions.
These gemstones include; Alexandrite, Andalusite, Iolite, Peridot, Rhodolite, Ruby, Sapphire, Spinel, Tourmaline and Tsavorite. Type 3 gemstones are usually heavily included. This group consists of Emerald, red Beryl and Rubellite Tourmaline.
The following chart summarizes the GIA Clarity Grades for colored gemstones.
Enhancements or Treatments:
A treatment or enhancement is any type of process, such as heating, oiling, waxing, irradiation, bleaching and dying, which alters the color or clarity of the gemstone. Nearly all gemstones available on the market today have been enhanced or treated in some way.
There are a variety of methods used to enhance the color or the clarity of the gemstone. Some of these methods are considered to be normal and accepted by the jewelry industry, such as heat treating to improve or change the color and clarity in Topaz and Sapphires, or oiling emeralds to improve their clarity.
Those gemstones that have not been enhanced or treated are very expensive. Some types of enhancements or treatments, such as heating and oiling, are both expected and accepted as normal by the jewelry industry because the change in the gemstone is permanent.
Unless a lab report is available to prove that the gem has not been heat treated, it should be assumed that the following gemstones are heat treated; aquamarine, sapphire, ruby, tanzanite, pink topaz, green tourmaline, blue zircon and carnelian.
Untreated gemstones should have an examination report by an independent certified gemological laboratory that clearly indicates that the gemstone is both natural (not man made) and untreated or unenhanced.