Demantoid is a sub-species of andratite which again is a sub-species of garnet. The other famous green garnet is Tsavorite, a grossularite and sister to hessonite. From this short intro we see that garnets are one of the mega families in the gem kingdom.
Demantoid began its public life as a predominately Russian product but it is today mined all over Africa and southern east Europe and the middle East. The ‘old’ Russian material, which the Czar was said to have loved so much, still commands the highest prices but has become very rare with few new mines and little estate sales. This gap was filled mainly by Namibian demantoids, though, in the beginning and absolutely unjustified, the market snubbed them for being less green (sometimes true) or less brilliant (not true at all, on the contrary).
Fresh demantoids now come from a variety of places, Pakistan, Madagascar, Iran, and even Turkey. Pakistani, Iranian and Afghani demantoids may even show the same horsetail inclusions that made Russians so expensive. Here is another example of subjectivity in the gem market. In 99% of all cases inclusions are deemed bad, yet in demantoid an asbestos-based impurity ups the price by a third, despite the unhealthy taste the word 'asbestos' leaves behind. So be it!
The one most obvious feature of demantoid is its Guinness Book of Records level fire. It beats any diamond before breakfast and adds fine greens for free. Other gems may rival their green, like emerald, chrome tourmaline or tsavorite, but never ever their adamantine sparkle. Our close-up images often catch little rainbows inside the gems, the hall mark of high dispersion or the splitting of white light into its main components (like rain splits sunlight - a rainbow). Only sphene or sphalerite surpass demantoid's brilliancy, and only the former is qualified for jewelry.
The rainbow fire magic aside, demantoids are a pest in front of the camera. Their purest green turns yellowish, their luster can't be shown (except on video which, forgive us, we don't do), they like to have inclusions that grow as soon as the camera comes out. Nevertheless, they are a fantastic sight in person.
Brilliancy oriented cuts, with as many facets as possible, preferable in precision quality, produce the most dazzling effect.
Almost as with rubies, size is critical in demantoid, one carat is a full weight, two carats are massive, and gems over four carats should be registered with the government as an addictive substance or weapons of mass distraction.
As a rule, the greener a demantoid the more expensive it is, however, a step sideways into yellow or brown, then probably called simply andratite will dramatically reduce your funds blood-letting, but provide as much fire and sparkle magic as green. And who says a sparkling rainbow in yellow is less attractive than a rainbow in green? Yet, as our prices attest, the market dictates that green-green is better. So be it!