Russian demantoid garnets are among the rarest and most valuable varieties of garnets. They were first discovered in the Ural Mountains of Russia in the mid-19th century and quickly became popular due to their unique green color and high brilliance.

The quality of Russian demantoid garnets is exceptional, and they are known for their high refractive index and dispersion, which gives them a fire and brilliance that is unmatched by other garnets. Their color ranges from a yellowish-green to a deep green, with some specimens displaying a golden "horsetail" inclusion that adds to their beauty.

The "horsetail" in Russian demantoid garnets refers to a unique inclusion that is characteristic of some specimens. This inclusion is a golden or reddish-brown needle-like structure that appears to radiate out from the center of the gemstone in a pattern that resembles a horsetail.

The horsetail inclusion is actually a form of asbestos known as chrysotile, which is embedded in the garnet crystal during its formation. It is considered to be a rare and desirable feature of demantoid garnets, and specimens with prominent horsetail inclusions are highly valued by collectors and gemstone enthusiasts.

The horsetail inclusion is often used as a diagnostic feature to help identify genuine Russian demantoid garnets, as this type of inclusion is not found in other types of garnets. While some other demantoid garnets from other localities may have similar inclusions, the horsetail inclusions found in Russian demantoid garnets are considered to be the most distinct and desirable.

There are several famous Russian demantoid garnets that have gained international recognition for their exceptional quality and beauty. Here are a few notable examples:

1. The Kazanjian Red Demantoid: This is a rare and stunning red demantoid garnet that was discovered in Russia in the 1990s. It weighs 5.05 carats and is considered to be one of the most valuable and unique demantoid garnets in the world.

2. The Demantoid "Star of David": This is a large demantoid garnet that weighs over 25 carats and has a unique six-pointed star pattern on the top of the gemstone. It was discovered in the Ural Mountains of Russia and is considered to be one of the largest and most beautiful demantoid garnets ever found.

3. "Star of Demark," which was discovered in the Ural Mountains and weighs over 12 carats.

Overall, Russian demantoid garnets are highly prized among collectors and gemstone enthusiasts due to their exceptional quality, rarity, and unique beauty.