Jewelry buyers beware: some fake gemstones have gotten so good, even gemologists are having trouble discerning real stones from their look-alikes. By definition, rhinestones are cut rock crystals, but just about any artificial gem--even glass and paste--can be referred to as a rhinestone. Fortunately, most of those are easy to spot. Less obvious are gemstone veneers, called composites. Sometimes a gem is real, but of a low quality, so it gets manipulated in order to look like a high-quality stone. There are a few things you can do to eliminate at least the most obvious fakes before paying an expert gemologist to inspect and appraise a gemstone.
Check the price tag and name. If the asking price is too good to be true, the gem is likely to be fake or stolen. Also, be wary of creative names. If there is a descriptor in front of a standard gem name; chances are it's not what the seller claims it is. For instance, a "Bohemian ruby" isn't a ruby at all, it's a type of quartz.
Examine the setting. Use a jeweler's loupe with a magnification of at least 10x and three lenses to correct for distortions. If the setting is high and solid, it's likely the stone has something to hide. Ask if there are any flaws beneath the setting. Even real gemstones sometimes have foil backings to bring out more color, but these aren't the highest quality stones so you shouldn't pay top dollar. Look for uneven color from the base to the top of the stone--it might have been dyed. Find out what material is used in the setting; if it isn't a precious metal, the stone probably isn't a real, quality gem.
Feel the gem. If you can scratch it with your fingernail or it doesn't feel.like a natural stone, it could be plastic or some other synthetic material. If a setting comes apart when you apply rubbing alcohol to it, it is a glued setting and therefore very unlikely that the gem is real.
Examine the gem. Look along the sides for tell-tale seams that indicate more than one gem glued together to appear to be one larger stone. These are known as "composites." Some "gems" are lesser rocks with gem veneers; seams should betray that, too. Now look within the gem itself. Real stones have cracks, internal pockets of air and even other materials in the stone itself, because it was made by natural forces. Manufactured stones are too perfect. If possible, compare it to a verified real gemstone of the same family. Uneven luster in the stone may indicate where cracks were filled to increase the weight and artificially enhance the value of the stone. Check a questionable diamond under UV light; cubic zirconia glows yellow under ultraviolet light, while real diamonds glow blue.
Go to an expert. Once you've established that a gem is not an obvious fake, it's worth spending a little money to have a gem inspected by a gemologist. Have at least two or three experts look it over if it is presumed to be a high-value piece.