The two main categories of gemstones used in jewelry are precious and semiprecious.
The classic list of precious stones for jewelry are diamonds, emeralds, rubies and sapphires. To this list can be added true alexandrites which are more valuable than diamonds. There are many more semiprecious stones.
Until a few years ago, there was no standard method of grading colored gemstones. The Gemological Institute of America (GIA) has now developed one that is complicated but workable. A gemologist trained and certified by them can now accurately grade colored gemstones and appraise their value using up to date price tables based on their grading system.
A major concern in buying jewelry with colored gemstones is their authenticity. Most of them have been altered to some degree to improve their color and/or clarity. Many of these alterations are common and universally accepted by the gem trade, such as the oiling of emeralds to hide the cracks common to this stone. Others are purely an effort to defraud the buying public.
A certified gemologist can spot the frauds. Manmade gemstones or synthetics, common with rubies, sapphires and emeralds, are the same material as the natural stones they synthesize and can be very attractive, but bring lower prices than the natural stones. They usually have fewer inclusions or defects than the natural stones and they can be spotted easily by a certified gemologist with the proper equipment.
The most common fraud is to misname a less valuable stone so it can be mistaken for a more valuable stone. Names like Arkansas diamond (quartz), Mexican jade (dyed green calcite) and water sapphire (iolite) are efforts to deceive. In the main, if a stone has an adjective, it is not what it sounds like.
However emeralds, rubies and sapphires are often correctly named after their country of origin. The book, “JEWELRY & GEMS, The Buying Guide” by Matlins and Bonanno and published by Gemstone Press contains valuable information on how to spot fakes.
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Better yet, have an independent appraisal by a GIA certified gemologist.