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|Rare Cat's Eye Alexandrite|
Strictly speaking, all gem-quality minerals are rare. That is because only a tiny fraction of the minerals found in the earth are sufficiently transparent and clean to be called gemstones.
But within the world of gem-quality stones, some gems are relatively common and and inexpensive. These are the stones you will find set in low-cost commercial jewelry, such as blue topaz, pyrope and almandine garnet, onxy and lower grade amethyst, citrine and peridot.
|Rare Gem Cobaltocalcite
On the other hand, there are certain gems which count as especially rare and are sought after by collectors and museums. These rare gems fall into two main categories. Some gems belong to rare varieties, where only a small number of specimens are known to exist. Any high quality specimen would count as rare. Then there are rare individual gems, distinguished by their exceptional color, clarity and size. Almost every gem variety has some especially rare specimens.
The rarest varieties tend to be obscure and little-known. They include unusual gems like jeremjevite, grandidierite, cobaltocalcite, poudretteite, cuprite, kornerupine and painite. Facet-grade specimens are exceptionally rare and mainly found in small sizes. These are gems for the dedicated collector and are seldom set in jewelry.
The most valuable rare gems tend to be the finest specimens of more common varieties such as diamond, ruby and sapphire. Usually these are very large gems of high quality, but not always. Red diamonds, for example, are among the rarest of all gems, but most are well under 2 carats. Fine unheated rubies almost never exceed 10 carats, and a clean pigeon-blood Burmese ruby over 5 carats can draw over $1 million at auction. Sapphires can be found in larger sizes, but excellent color and clarity and absence of heat treatment are more of a factor in rarity than mere size.
|Rare Black Star Sapphire
with Golden Star
Gems that display unique phenomena such as asterism (the star effect), chatoyancy (the cat's eye effect) or color-change form a special class of rare gems. A fine royal blue sapphire that displays a clear star is a rare and valuable gem, especially if it is very large (over 10 carats) and shows some translucency. Translucent star rubies with good body color are rare in any size. A color-change alexandrite that also displays a cat's eye is doubly rare.
In some cases rarity is connected with specific geographic origins. Demantoid is a rare variety of garnet, but the demantoids from Russia have a special status that is not shared by the more ordinary specimens from Namibia and Madagascar. The Russian demantoids are colored by chromium and display a vivid green color. Internally the Russian gems display fine needles in a radiating pattern known in the trade as a horsetail inclusion. Only the Russian demantoid display this unique feature, which is prized by collectors.