Even for professional gemologists, it can be difficult to tell a fake gemstone from a real one. With sophisticated technologies capable of manufacturing man made stones that not only look like the real thing, but have also been created using the same general process as the one used by nature, it has become increasingly complex to differentiate between real and fake gems.
The two major categories of man made stones are glass stones and synthetic. Unlike glass stones, synthetics are normally created in a lab. Of the two, glass stones are far easiest to recognize. They may have tiny bubbles or a lot of little scratches. The cut may also have rounded edges.
Synthetic gemstones are created using two different techniques. They can be grown in a lab using the same materials and processes used by nature. They can also be simulated using different materials, which result in a different color, hardness and luster. With synthetic gems, you may see a scissor cut in the shape of an X. Or, they may have little grooves on them. Real gems will have inclusions (internal air bubbles, cracks and non-gem minerals) while synthetic gems usually won't.
There are also techniques for making stones either partially from a real gem, or to make the gem appear better than it really is. These include filling holes, putting foil behind the back of the setting, composites, closed-back settings and heat treatment
Filling holes can add weight and thereby expense to a real gem. They can also make a stone appear better than it is. This can be uncovered by looking for uneven luster.
Placing foil behind a stone is a way to enhance its brilliance. A closed back or bezel setting may be used to conceal this; it can also be used to hide a flaw in the stone.
Composites can either be made by gluing two or more pieces of the gem together, or by gluing a gem to something such as glass or a synthetic stone. When dipped in rubbing alcohol, these tend to become unglued.
Heat treatments can be used to change or enhance the color of a gem. For some gems this is an almost routine process.
For the inexperienced, these differences can be difficult to spot. There are other things, however, that should make you suspect that the stone is not a natural one.
If the price is too good to be true, it probably is not a real gem. Also, a gem is always called only by the name of the stone. For example, a "Bohemian ruby" is not a ruby at all; it is rose quartz. If the stone is "cultured" or "created" then it is not a natural gem.
To protect yourself from fraudulent gem dealers, always go to one you know to be reputable, or is certified by an organization such as the Gemological Institute of America. Ask if it is a natural stone, and if it has been modified in any way. Have the jeweler put into writing whether or not the stone is real and natural, and if it has been altered.
In the end, unless you are looking for an investment, what really matters is whether or not you like a piece of jewelry. You just don't want to pay more for it than it's worth. Ultimately, only an experienced certified gemologist can make that determination.