All gemstones are rare; some are just harder to find than others.
In fact, there is no consensus on what is the rarest mineral or the rarest gemstone because there is no consensus on the definition of "rarity," according to the Gemological Institute of America.
However, many of the stones in this series come from only one or two localities in the entire world, so in that sense, they are scarce.
Pink Star Diamond
In the image above, model Annabeth Murphy-Thomas poses with the Pink Star diamond at Sotheby's auction house in central London. The diamond was put up for auction in Geneva on Nov. 13, 2013, at $60 million, an already record price for a gemstone, and sold for $83 million. Diamond cutter Isaac Wolf of New York purchased the Pink Star diamond ring, and renamed it the Pink Dream. The diamond measures 1.06 inches by 0.81 inches (2.69cm by 2.06cm).
Opal is Australia’s national gemstone, and black opal is the rarest and most valuable of its kind, at times selling at prices that rival the best diamonds. The stone must have a rich, black background, but base colors come in all shades of gray, which is why opinions vary on what is a "true" black opal.
Found in the Lightning Ridge area in northwestern New South Wales, black opals are natural, solid stones that absorb scattered white light, giving it brilliant spectral colors.
The Gemological Institute of America (GIA) calls musgravite "a rarity among the rare... a particular gem on our research examination 'want list.'"
A very close relative of another hard-to-find gemstone, taaffeite (and often misidentified as such), musgravite was first discovered in 1967 in the Musgrave Range of South Australia.
Facet grade -- the baseline measurement of how clean cut a sellable stone must be -- for musgravite was not reported until 1993. As of 2005, there were only eight musgravite specimens in the world.
Discovered in 1951 in Mogok, Burma, painite was once considered the rarest mineral on Earth. For decades, only two crystals were known to exist.
It didn't obtain official gemstone status until 1957 when the British Museum conducted X-ray analysis on a sample. In 1979, a third crystal was recovered by the GIA.
Today, more than a thousand crystals and crystal fragments have been found. However, only a small percentage of the rough are suitable for sale.
Painite is made up of aluminium, calcium, boron, zirconium and oxygen. It gets its orange-red to brownish-red color from trace amounts of iron.
Jeremejevite is an extremely rare, aluminium borate mineral. It was discovered in the late 19th century and named after Pavel V. Jeremejev, a Russian mineralogist and engineer.
Until recently, the only two known localities for jeremejevite were Mt. Soktuj in the Transbaikal region of Russia and Cape Cross, Swakopmund, Namibia.
Not much is known about jeremejevite. The color is typically aquamarine, but other records show the mineral can also be dark blue, pale yellow-brown or colorless.
Red diamonds, just like any other diamonds, are made of compressed carbon. However, the brilliant red color in these diamonds is formed from a structural defect in the crystal lattice structure, which is why they are the rarest of the colored diamond collection.
Only a handful have ever received the grade of "Fancy Red," meaning that they are pure red with no modifying color. Most are sold at market for millions of dollars.
The Argyle mine in Australia is the primary producer of pink and sometimes red diamonds.
This transparent, blue gem first turned up in 1962 and has been found scattered throughout northern Tanzania in Africa.
Ranging in color from light blue to pure blue to dark violet-blue, the deepest hues are valued most.
Made popular by jewerly giant Tiffany & Co. in 1968, Tanzanite has seen wild price fluctuations over the years. Tanzania's violent political, social and economic conditions have made it difficult at times to mine the mineral. However, the nation remains the gem's only known source.
Although "red emerald” is its snazzy marketing name, and it was originally called "bixbite," this mineral goes by the name "red beryl" today.
The brilliant red-purple color is not a trick of the light. The stone's actual chemistry is distinctive and separate from other beryls. It is found along fractures in topaz rhyolites. The gem crystallizes when rhyolite-derived gases, vapors from heated groundwater, and preexisting minerals and volcanic glass in the rhyolite react all at once.
There is only one known commercial production of gem-quality red beryl in the world: the Ruby Violet (or Red Beryl) mine in the Wah Wah Mountains of Beaver County, Utah.
Still one of the rarest gems known today, this pinkish mineral was named after the Poudrette family, owners and operators of a quarry near Mont Saint-Hilaire, Quebec, Canada, where poudretteite was first found.
It was discovered as a few tiny crystals during the mid-1960s, but wasn't recognized as a new mineral until 1986. The first documented gem-quality specimen of poudretteite wasn't discovered until 2000, when it was found in Mogok, Burma.
This remarkable, flawless 9.41 carat poudretteite gem from Burma is truly one-of-a-kind. It is considered to be one of the largest -- if not the largest -- faceted poudretteite in existence.