Demantoid garnets are very rare stones that are only mined a few places in the world. Demantoids are valued for their rarity, their beautiful sparkle and their unique form.
Look for inclusions. True demantoid garnets have wispy, very pronounced inclusions that look like fine hairs. These are called horsetail inclusions, and dramatically increase the price of a demantoid because they are so beautiful and unusual. A demantoid may have one of these inclusions or many.
Ask about the country of origin. Real demantoid garnets come from the Ural Mountains in Russia. There are gemstone "seams" in Nambia and even Korea that produce green garnets that are called demantoids, but these tend to be flawless, which makes them worth much less than the real thing. However, they can be beautiful in jewellery, and are much less expensive.
Evaluate the colour. Demantoids range from yellow-green to brownish-green. The most valuable variety is a clear, emerald green. If a stone has flashes of blue, red or purple, then it is likely not a demantoid garnet. Demantoids often have a golden glow that shines through the green colour.
Weigh the stone. Demantoid garnets tend to be quite small, and usually weigh less than 2 carats after cutting. In fact, anything even close to 1 carat is extremely valuable. A large gemstone is very unlikely to be a true demantoid garnet, but may be either a clever fake or a more common verdelite garnet.
Examine the brilliance. Demantoids are known for their intense, pervading brilliance. However, it is important to factor in the presence of the horsetail inclusions. When you are evaluating a potential demantoid garnet, remember that its brilliance may be tempered by the inclusions. If this is the case, you will need to consult a jeweller to determine exactly how to rate the stone's sparkle.
Work with a licensed jeweller when you are investing in a demantoid garnet. They are very rare and can be extremely expensive, so you do not want to accidentally invest in a false or misnamed stone.