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Gemstones may be measured by weight, size, or both. The basic unit for weighing gemstones is the carat, which is equal to one-fifth (1/5th) of a gram. Carats are divided into 100 units, called points. For example, a half-carat gemstone would weigh .50 carats or 50 points. When gemstones are measured by dimensions, the size is expressed in millimeters (for example, 7x5 millimeters).

Gemstone treatments or enhancements refer to the way some gems are treated to improve their appearance or durability, or even change their color. Many gemstones are treated in some way. The effects of some treatments may lessen or change over time and some treated stones may require special care. Some enhancements also affect the value of a stone, when measured against a comparable untreated stone.

Jewelers should tell you whether the gemstone you're considering has been treated when: the treatment is not permanent; the treated stone requires special care; or the treatment significantly affects the value of the gemstone.

Some common treatments that you may be told about and their effects include:

Heating can lighten, darken or change the color of some gems, or improve a gemstone's clarity.
Irradiation can add more color to colored diamonds, certain other gemstones and pearls.
Impregnating some gems with colorless oils, wax or resins makes a variety of imperfections less visible and can improve the gemstones' clarity and appearance.
Fracture filling hides cracks or fractures in gems by injecting colorless plastic or glass into the cracks and improves the gemstones' appearance and durability. Diffusion treatment adds color to the surface of colorless gems; the center of the stone remains colorless. Dyeing adds color and improves color uniformity in some gemstones and pearls. Bleaching lightens and whitens some gems, including jade and pearls.

Diamonds
A diamond's value is based on four criteria: color, cut, clarity, and carat. The clarity and color of a diamond usually are graded. However, scales are not uniform: a clarity grade of "slightly included" may represent a different grade on one grading system versus another, depending on the terms used in the scale. Make sure you know how a particular scale and grade represent the color or clarity of the diamond you're considering. A diamond can be described as "flawless" only if it has no visible surface or internal imperfections when viewed under 10-power magnification by a skilled diamond grader.

As with other gems, diamond weight usually is stated in carats. Diamond weight may be described in decimal or fractional parts of a carat. If the weight is given in decimal parts of a carat, the figure should be accurate to the last decimal place. For example, ".30 carat" could represent a diamond that weighs between .295 - .304 carat. Some retailers describe diamond weight in fractions and use the fraction to represent a range of weights. For example, a diamond described as 1/2 carat could weigh between .47 - .54 carat. If diamond weight is stated as fractional parts of a carat, the retailer should disclose two things: that the weight is not exact, and the reasonable range of weight for each fraction or the weight tolerance being used.

Some diamonds may be treated to improve their appearance in similar ways as other gemstones. Since these treatments improve the clarity of the diamond, some jewelers refer to them as clarity enhancement. One type of treatment - fracture filling - conceals cracks in diamonds by filling them with a foreign substance. This filling may not be permanent and jewelers should tell you if the diamond you're considering has been fracture-filled.

Another treatment - lasering - involves the use of a laser beam to improve the appearance of diamonds that have black inclusions or spots. A laser beam is aimed at the inclusion. Acid is then forced through a tiny tunnel made by the laser beam to remove the inclusion. Lasering is permanent and a laser-drilled stone does not require special care.

While a laser-drilled diamond may appear as beautiful as a comparable untreated stone, it may not be as valuable. That's because an untreated stone of the same quality is rarer and therefore more valuable. Jewelers should tell you whether the diamond you're considering has been laser-drilled.

Imitation diamonds, such as cubic zirconia, resemble diamonds in appearance but are much less costly. Certain laboratory-created gemstones, such as lab-created moissanite, also resemble diamonds and may not be adequately detected by the instruments originally used to identify cubic zirconia. Ask your jeweler if he has the current testing equipment to distinguish between diamonds and other lab-created stones.

Pearls
Natural or real pearls are made by oysters and other mollusks. Cultured pearls also are grown by mollusks, but with human intervention; that is, an irritant introduced into the shells causes a pearl to grow. Imitation pearls are man-made with glass, plastic, or organic materials.

Because natural pearls are very rare, most pearls used in jewelry are either cultured or imitation pearls. Cultured pearls, because they are made by oysters or mollusks, usually are more expensive than imitation pearls. A cultured pearl's value is largely based on its size, usually stated in millimeters, and the quality of its nacre coating, which gives it luster. Jewelers should tell you if the pearls are cultured or imitation.

Some black, bronze, gold, purple, blue and orange pearls, whether natural or cultured, occur that way in nature; some, however, are dyed through various processes. Jewelers should tell you whether the colored pearls are naturally colored, dyed or irradiated.

How to buy jewelry - part I
Jewelry shopper's checklist