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The only real way to determine if an emerald is real, is to take it to a Reputabel Jewler.
However, there are several "folk" tests that will give a good indiction:
1. If you drop a real emerald into a pot of boiling water, it will radiate green light, while an imitation glass one will not.
2. If a real emerald is placed on the eyes, it will feel cool; a fake will heat up quickly. (this is based in truth on the cold/heat conduction of different materials. rocks and gems, granite, marble, etc...will feel cooler to the touch than glass. The eye lids are a sensitive area of the body. You might get the same sensation by using the side of our cheek).
3. A drop of water on a real gemstone will tend to retain its shape. The water drop, depending on its size, may not be significantly affected while moving the gemstone around. However, on a glass piece, the water will spread out emediately. (this test is based on the surface tension of water and the natural "rock/gem" microscopic contours of the surface. Glass is flat, and smooth, allowing the water to smooth out quickly.
Today, there are many "laboratory" gems, that are in essence real emeralds. They have a more pure look from the natural.
Take your gem to the jewer for a difinitive determination.