Demantoid

The demantoid is one of the most brilliant gemstones that exist, yet until recently it was little known except among collectors and gemstone lovers. Strictly speaking it is a green garnet, or rather the star of the green garnets. Not without reason does it bear a name which means 'diamond-like'. The name comes from the Dutch and makes reference to the outstanding quality of this gem, its incomparable brilliance and fire. Some gemstone lovers claim that a demantoid will continue to glow even in the shade.
Why the horsetail influences the value of a demantoid
Demantoids from Namibia come in shades from a vivacious light green to an intense blue-green. They have a striking brilliance. Thanks to their hardness of just under 7 on the Mohs scale, they are well suited to being used in jewellery. However, they do lack one feature by which the true demantoid had always been able to be identified through the microscope: 'horsetail inclusions'. These golden brown crystal threads of chrysotile, mostly appearing to radiate out from the centre of the stone, had previously occurred in almost all demantoids. But - more's the pity - they were missing in the relatively inclusion-free gems from Namibia. These horsetail inclusions were not only typical of the demantoid; they could even increase its value if they were pronounced. That may sound surprising, since as a rule inclusions, which can impair the transparency of a gemstone, are not a welcome sight. But with the demantoid's 'horsetail inclusions' it is a different matter. A beautiful, well formed inclusion can increase the value of the gemstone considerably, a good many collectors being prepared to pay a higher price.