The line between genuine and fake gemstones is often very blurred. I recently started jewelery-making, crafting simple necklaces, earrings, and rings, many of which feature gemstone beads. The other day my father asked about the stone in a ring I had just made. I informed him that it was amethyst. He immediately insisted that it couldn't possibly be real amethyst - after all, real amethyst costs a fortune! He also said the same thing about some pearls that I had purchased, as well as some turquoise. Personally I feel that people are often too quick to jump to the conclusion that gemstones are not real. One reason for this is that they do not realize the great level of natural variation that exists in many gemstones.
For example, the turquoise beads that I had were a lovely grass-green color with a slight brown matrix on them. My father could not believe that this was really turquoise, simply because its color was not turquoise! That would seem like a necessary trait of all real turquoise, right? But green turquoise is not at all uncommon in areas such as Arizona and Nevada. The green color is simply due to higher levels of iron in the stone.
Also, many natural gemstones are often altered for commercial appeal. For example, those pearls I mentioned earlier are called peacock pearls, due to the fact that they have been dyed to have an iridescent blue/purple color. Does this make the pearls fake, or less than genuine? I think not. I have no problem using dyed gemstones in my work. In my opinion, such stones simply offer a wider variety for jewelery makers and buyers to choose from. As long as the seller makes it clear that they are selling a piece that has been altered from its natural state, I see no harm.
In addition to the practice of drastically altering a natural gemstone's appearance, there are also methods for enhancing the natural beauty of a gemstone. Many gemstones are given what is called a "color shot", which either enhances its natural color, or alters it slightly to make it look more exotic. For example, turquoise and related stones are often given color shots to either achieve a more vivid blue, or an unnatural exotic color, such as bright purple. With most stones, if there is any alteration, I prefer an enhancement of the stone's natural beauty to the creation of an unnatural color, but many find the latter process appealing as well. Simply put, it is important to just know exactly what you want as a buyer.
In order to know what you want, you should be equipped with at least a general knowledge of the stone that you are looking for. If you are looking for turquoise or amethyst for example, be aware that there is a variation in the appearance of these stones, particularly their color. There are such gemstones as green amethyst, and green and yellow varieties of turquoise. Without the knowledge of such variations, one might be quick to decide that a gemstone is indeed fake and a seller illigitimate.
It is also important to have a good, working knowledge of gemstones in general - not just the gemstones that you are interested in. This is because if you ever encounter a seller who is truly trying to fool you, he or she will likely be trying to sell you another, cheaper gemstone, which has a similar appearance to the one you are looking for - either because of natural similarities or lab-created similarities. For instance, purple fluorite can often be mistaken for genuine amethyst.
In particular, be very careful when looking for emeralds, rubies, sapphires, and diamonds. These are some of the most valuable gemstones, and therefore the most common fakes. Also realize that the value of some gemstones may not always be as great as you would expect (as my father so well demonstrates). Amethyst, for example, used to be much more valuable and prized than it is today. It has since become more common after the discovery of great amounts of amethyst in South America.
Speaking of location, that is another important thing to be aware of when looking for real gemstones. Know where certain gemstones are found in the world. While some occur naturally all over the world, others have very few places of origin. Lapis lazuli, for example, is found almost exclusively in Afghanistan. A seller should be well aware of such facts, and if you are as well, you will be difficult to fool.
So in looking for gemstones, be vigilant, sure of what you are looking for, have a good general knowledge of the specific stone as well as gemstones in general, and in the back of your mind, remember that good deals do exist, although if it sounds too good to be true to your gemstone-savvy ears, it probably is!