Unscrupulous salespeople have devised many ways to trick the unsuspecting purchaser into buying glass or synthetic material disguised as a precious or semiprecious stone. Fortunately, the shopper who knows what to look for can often identify fake stones by examining them carefully. Though some particularly good imitations may require a professional to identify, an amateur who has done his homework can evaluate the stone's perfection, price point and hardness to tell whether or not it is a true specimen.
Examine the item for tiny bubbles or scratches. True gemstones do not have these, but glass may.
Look at the facet cuts. If they are rounded, the piece is glass. Gemstones have flat, precisely cut facets.
Consider the price logically. If the item is rich in color, clear (if the stone is meant to be clear), cut perfectly evenly and looks like an expensive jewel but costs much less than you expect, it is likely to be glass.
Research the qualities of the stone and learn its hardness on the Mohs scale. Test the stone against the item in a Mohs kit for the appropriate hardness. Glass is a six or seven on the scale, so if you are buying a sapphire, which is a nine, scratch it with topaz, which is an eight. If the topaz scratches, the sapphire is real. If the sapphire scratches, it is actually glass.
When buying a diamond, ask for a certificate of authenticity from the Gemological Institute of America or an appraiser from the American Society of Appraisers.