A real emerald will radiate green light when placed in a glass pot filled with water while an imitation will not. Another way to tell the difference is to place a drop of water on the stone. If it spreads out, the emerald is not real.
Emeralds must always be medium to dark green. Lighter colors are not considered emeralds, but instead are called green beryl. As with other gemstones, the value of an emerald decreases if there are visible flaws that affect the stone's clarity and transparency. Generally, the larger the emerald, the more it will cost. Emeralds are difficult to cut because they contain many very small cracks which make them prone to breakage.
Synthetic, or man-made, emeralds generally sell for between $500 and $1,000 per carat, which is extremely low for this type of stone. For many, this can be a sign that the stone in question is not a genuine emerald. Many sellers will try to pass off a synthetic stone as real because the chemical composition of the two stones is very similar.
Besides the tests listed above, the only real way to find out if an emerald is natural or synthetic is to take it to a gemologist for testing.