Demantoid is the green gemstone variety of the mineral andradite, a member of the garnet group of minerals. Andradite is a calcium- and iron-rich garnet. It is the most expensive and rare of garnet gemstones.

appearance demantoid by definition is always green, but the exact shade ranges from a very light yellowish peridot green to nearly the color of a fine emerald. many stones have a brownish cast. stones with more intense green coloration are more highly valued, but lighter stones display substantially more fire. the choice of stone color or fire can therefore be a matter of personal preference, with some preferring the less valuable but more lively yellowish-green stones. its dispersion (0.057) is unusually high, and this is often visible as "fire" (rainbow-coloured flashes of light), although in some cases the stone's green body colour can render this effect less noticeable. demantoid also has a high refractive index of 1.80 to 1.89. demantoids are generally small, with finished stones over one carat (200 mg) uncommon and stones over two carats (400 mg) quite rare. clean stones over five carats (1 g) are considered world-class. russian demantoid nearly always contains inclusions of byssolite and/or chrysotile[4], both of which are types of asbestos. these inclusions are feathery golden threads that tend to curve and resemble the tail of a horse, and are therefore referred to as horsetail inclusions. in gemmology, the presence of such inclusions is regarded as diagnostic for natural demantoid. unlike most inclusions which reduce the value of a gemstone, aesthetically pleasing horsetail inclusions can substantially increase the value of a stone. namibian demantoid does not contain horsetails (although at least one contradictory report is known[5]). even horsetails which are not visible to the naked eye are valuable in identifying the origin of a stone. the microstructure of some demantoids is believed to be affected by the presence of 'horsetails'. demantoids containing horsetails tend to form as pebble-like nodules without well-defined crystal faces[6] (the 'horsetail' typically originates towards the centre of the nodule, with the fibres branching out and radiating towards the surface), whereas horsetail-free demantoids from other sources frequently display flat crystal faces