The rarest gemstones are minerals known from only one or a few deposits, or that form transparent, cuttable crystals only in tiny amounts. Painite, red beryl, taaffeite, musgravite, jeremejevite, grandidierite, poudretteite and benitoite are the names collectors mention most, along with natural red diamond and fine alexandrite. Most of these stones are small, and many people have never seen one outside a museum or a collector's case.
Rarity here means rare in gem quality. Some of these minerals exist as opaque grains or tiny crystals in rock, while faceted stones of any size number in the hundreds or fewer.
Painite
Painite is a calcium zirconium aluminum borate first identified in Myanmar in the 1950s and named after Arthur C. D. Pain, the British gem dealer who brought the first specimen to the attention of mineralogists. For several decades only a handful of crystals were known, and painite often appeared in record books as the rarest mineral on Earth. From the early 2000s more material turned up in the Mogok area, the same region known for Burmese ruby, so painite is now less rare than it was, though clean faceted stones remain scarce. It is usually reddish brown to orange-red, with a hardness of about 8.
Red beryl
Red beryl is the same mineral species as emerald and aquamarine, colored red by manganese. The only source that has produced gem crystals is in the Wah Wah Mountains of Utah, where it formed in cracks in rhyolite, a volcanic rock. Most crystals are tiny, and faceted stones above one carat are very uncommon. It was once sold as "bixbite," after the mineral collector Maynard Bixby, but the trade now avoids that name because it is easily confused with bixbyite, a different mineral. Red beryl rates 7.5 to 8 on the Mohs hardness scale and usually contains inclusions.
Taaffeite and musgravite
These two close relatives are magnesium beryllium aluminum oxides. Taaffeite has a curious history: in 1945 Count Richard Taaffe, a gemologist in Dublin, noticed that one stone in a parcel of cut spinels showed double refraction, which spinel never does. Testing showed it was a new mineral, and taaffeite became one of the very few minerals first identified from a faceted gem. Most gem taaffeite comes from Sri Lanka and Tanzania and ranges from pale mauve to lavender and pink. It looks so much like spinel that more stones may still sit unrecognized in old spinel parcels.
Musgravite was described in 1967 from the Musgrave Ranges of South Australia. Gem-quality musgravite appeared later, mainly from Sri Lanka and in smaller amounts from Madagascar, Myanmar and Tanzania. It is usually grayish green to grayish purple. Both minerals are hard, around 8 to 8.5, and separating them from each other usually takes advanced lab testing such as Raman spectroscopy.
Jeremejevite
Jeremejevite is an aluminum borate first described in 1883 from Siberia and named after the Russian mineralogist Pavel Jeremejev. Gem crystals are mostly from Namibia, especially the Erongo region. They range from colorless to pale blue and cornflower blue, often as slender six-sided prisms. Jeremejevite has a hardness of about 6.5 to 7.5, and clean faceted stones are rarely more than a few carats.
Grandidierite
Grandidierite is a magnesium aluminum borosilicate first described from Madagascar in 1902 and named after Alfred Grandidier, a French naturalist who studied the island. For most of the 20th century it was known mainly as an opaque mineral. Transparent bluish green material suitable for faceting has been found in Sri Lanka and, in the 2010s, in Madagascar. It shows strong pleochroism: the color shifts between shades of blue-green and a near colorless tone as the stone is turned. The gems of Madagascar page covers the island's other gem deposits.
Poudretteite
Poudretteite, a potassium sodium boron silicate, was first found in the 1960s at the Poudrette quarry on Mont Saint-Hilaire in Quebec, a locality famous for unusual minerals. It was formally described as a new mineral in 1987. Gem-quality pink crystals turned up in the Mogok region of Myanmar in 2000, and faceted poudretteite has been extremely scarce ever since. It is soft for a gem, about 5 on the Mohs scale, so it is a collector's stone and seldom worn.
Benitoite
Benitoite is a barium titanium silicate discovered in 1907 in San Benito County, California, and it became the California state gem in 1985. Gem crystals come almost entirely from that one area. It is usually sapphire blue, glows bright blue under shortwave ultraviolet light and has dispersion close to that of diamond, giving faceted stones visible fire. Most cut benitoites weigh under one carat. Hardness is about 6 to 6.5.
Red diamond
Natural diamonds come in many colors, but a pure red is the rarest. Like pink diamonds, red diamonds owe their color to distortion of the crystal lattice (plastic deformation) deep in the earth, not to a chemical impurity. GIA gives only a handful of stones the grade Fancy Red, the term used when no modifying color is present. The Moussaieff Red, a little over 5 carats and found in Brazil, is often cited as the largest known. Read more in fancy colored diamonds.
Alexandrite
Alexandrite, the color-change variety of chrysoberyl, was first found in Russia's Ural Mountains in the 1830s. Later deposits in Brazil, Sri Lanka and East Africa made it more available, but stones with a strong, clean change from green to red, good clarity and a size above a carat or two are still rare. Alexandrite is far less rare than painite or red beryl, though it sells to a much wider market.
Rarity versus value
A rare mineral is not automatically an expensive gem. Price depends on demand as much as on supply, and demand depends on beauty, durability, size and how well buyers know the stone. Fine ruby, emerald and sapphire are much more common than poudretteite, yet a top ruby can sell for far more per carat. Many rare species are pale, small, soft or heavily included, so they appeal mainly to collectors of unusual minerals rather than jewelry buyers.
Rarity also changes. A new find can turn a mineral known from a few crystals into one sold at gem shows, as happened with painite. Gems like tanzanite, known from a single small area of Tanzania, could become rarer if that source is worked out. For how minerals are grouped into species and varieties, see how minerals are classified.
Frequently asked questions
What is the rarest gemstone in the world?
There is no single agreed answer. Painite long held the title, and red beryl, taaffeite, musgravite and natural red diamond are often named, depending on whether you count all known crystals or only gem-quality ones.
Is red beryl the same as emerald?
Both are beryl, so they share the same chemistry and structure. Emerald is green from chromium or vanadium, while red beryl gets its color from manganese.
Are rare gemstones good for jewelry?
Some are. Taaffeite and musgravite are hard enough for rings, while poudretteite and benitoite are softer and better suited to pendants or collections.
Why are the rarest gems not always the most expensive?
Price follows demand as well as supply. Well-known gems such as ruby and sapphire have far more buyers than obscure minerals do.