All gemstones are rare. That's part of what makes them gemstones. However, some gemstones are much harder to find than others. Some gemstones can only be found in a small area. In some cases, a specific variant of a well-known gemstone is extremely rare. The price on rare gemstones does not always reflect its real rarity, partly because a gemstone which is too rare does not have high enough recognition to build a market.
As of the time of writing, this light pink gemstone may be the rarest gem in the world. It was not even identified as a new mineral until 1986, while the first best gem-quality specimen was not found until 2000. Poudretteite can still be found in Quebec, where it was originally identified, but most new gemstones come from Mogok, Myanmar.
Painite is usually an orange or reddish-brown gemstone, although it can sometimes be pink. Until recently, only 2 faceted painite crystals were known to exist. A new source was discovered in 2002, and the original source was identified a few years later. All known sources are in Myanmar. As a result, painite is less rare than it used to be. However, high quality facet material is still extremely hard to find.
This gemstone ranges from colorless to pale yellow or blue. In extremely rare cases, jeremejevite can also be a deep blue. It has been found in only 3 places: Namibia, the Pamir Mountains of Tajikistan, and the Eifel district in Germany.
Nearly all clinohumite takes the form of tiny yellow-orange grains. Gem-quality crystals are extremely rare. Only 2 sources are known: the Taymyr region of Siberia, and the Pamir Mountains of Tajikistan. Gemstones from the Pamir Mountains are usually lighter in color and slightly harder than the dark reddish stones from Taymyr.
While it has been in previous use as a silicate mineral, serendibite is fairly new as a gemstone. Most gem-quality crystals are dark gray or purplish-gray to black, although a few can be found in colors ranging from dark blue and green all the way to yellow.
For the most part, gem-quality stones can only be found in the Mandalay Division of Burma and at Gangapitiya, Sri Lanka, which gives the gem its name. An occasional gem-quality stone can also be found in northern Russia and Canada, in the quarries of New York and California, and in Tanzania.
Until recently, the only known source for gem-quality benitoite was San Benito County, for which it is named. California's state gem is almost completely unknown outside the state of California, which is the primary reason why it is priced so low. In fact, this is literally used in many gemology courses as a textbook example of the disconnect between rarity and price. Benitoite may be rare, but the knowledge and demand outside the immediate area where benitoite is mined is nearly non-existent.
This purple-blue gemstone is not often cut for jewelry, although it is well suited for that purpose. Instead, it is usually kept in its natural form, still in its original matrix.
Although alexandrite itself is not especially rare as gemstones go, most forms of alexandrite vary between light shades of greens, yellows, reds, or purples. The most desirable color change is a vivid green by daylight, and bright red or purple red under incandescent light. A few Tanzanian alexandrites have this property, as well as some Ural Mountain alexandrites. However, very few Russian stones are produced today, placing the Ural Mountain alexandrite among the world's rarest and most desirable gemstones.
Also known as red emerald, red beryl is sometimes referred to as bixbite, but this name is being phased out to avoid confusion with bixbyite, both of which were discovered by the same minerologist, Maynard Bixby. It has the same chemical structure as standard emerald, morganite, and other forms of beryl. The rich red color of red beryl is caused by manganese ions.
Red beryl is by far the rarest of all the beryls. It is almost never found outside Utah and New Mexico. Most gem-grade red beryl comes from Utah's Wah Wah Mountains.
While blue sapphires are fairly common as gemstones go, Kashmir sapphires have a unique quality of blue which is sometimes described as blue velvet. Nearly all known Kashmir emeralds were discovered between 1880 and 1900. The original source for these stones, high in the Himalayas, has been depleted. Very few new Kashmir sapphires have been found.
Although diamonds are believed to be extremely rare, this is not the case. After all, many of your married female friends probably have at least a diamond ring. However, the red diamond is genuinely rare. Only a few dozen are known to exist, most of them half a carat or less in size. Even the largest known red diamond, the Moussaieff Red Diamond, is just 5.11 carats.
Red diamonds are so rare because their color doesn't come from chemical impurities or radiation exposure. Instead, the deep red color of red diamonds is believed to be caused by tiny defects in the crystal lattice.
For something completely different
At the other end of the scale, peridot may be much less rare than gem enthusiasts used to think. Its mineral form, olivine, is now believed to make up most of the mantle of rocky planets, including the Earth itself. The meteorites known as pallasites also contain olivine. It has even been found in the halos of distant stars, making it potentially one of the most common minerals in the universe.
However, much of that olivine is inaccessible. When it reaches the surface of the earth, it weathers fast. Cross-sectioned, polished pallasites are still something rare and beautiful. A few pallasites even contain a carat or so of peridot, making these extraterrestrial gems rare indeed.