The most expensive types of gems are usually the rarest. A single gem takes millions of years to form, and then humans must find, mine and refine it before it can be sold as jewelry or ornament. With certain rare gems, only a few known examples have been found and are often not available for commercial sale. These often command the highest prices. The size of gems also determines the price point. Fine specimens of common jewelry gems, such as diamonds and rubies, weighing a carat or more carry a high price, too.

Tanzanite hails from Northern Tanzania and is on the gem endangered species list. The Discovery Channel reports it may be mined out within 20 to 30 years. Scarcity and limited supply thus makes the blue-purple stone highly valuable at $600 to $1,000 per carat. Taaffeite is another purplish stone that is also found in Tanzania as well as Sri Lanka. Because only a few have ever been discovered, it brings a high price of $1,500 to $2,500 per carat. Musgravite is categorized in the same gem family as Taafeite and ranges in color from greenish gray to purple. Limited specimens have been found in Greenland, Sri Lanka, Madagascar and Tanzania, as well as in Southern Australia. It's still rare, though, and brings about $35,000 per carat.

Opals aren't usually ranked among the most expensive gems, unless they're of the black variety. This gemstone primarily comes from one mine in New South Wales, Australia. They reveal streaks of color, resembling the glow of fire, on their black base. Because they're relatively rare, black opals command $2,300 or more per carat. Benitoite comes mainly from California and features a bluish-purple hue. Due to its rarity, collectors flock to this gem, and it's sometimes cut into jewelry -- usually in sizes less than a carat. It brings between $3,000 and $4,000 per carat.

Red beryl, sometimes called scarlet emerald, comes from Utah and New Mexico and is considered 1,000 times more valuable than gold, bringing in as much as $10,000 per carat. Alexandrite, originally discovered in the Ural Mountains of Russia, has also been found in Brazil, East Africa and Sri Lanka. This color-shifting stone brings about $12,000 per carat. Jadeite, no relation to the semiprecious jade, is quite rare and has only been found in Myanmar. In 1997, a jadeite necklace sold for almost $10 million. Today, the highest quality jadeite stones sell for more than $20,000 per carat. Painite is another of the Earth's rarest gems and is worth between $50,000 and $60,000 per carat. Painite, found in Myanmar, appears orange-red and is rarely cut into wearable gems.

Rubies and sapphires are often considered valuable gems, but they only command impressively high prices when presented in sizes that exceed several carats and are in flawless condition. For example, the Logan sapphire, kept at the Smithsonian, is 422 carats and nearly priceless due to its size. Small rubies are relatively plentiful in the gem world, while the largest stones are extremely rare and bring an impressive price. Diamonds are rare on Earth, but in the jewelry and gem world are rather commonplace. The price of diamonds depends on the quality of the stone and the size. Certain diamonds, such as the 59.6-carat Pink Star diamond, are among the rarest and most expensive gems in the world. In 2013, the Pink Star diamond earned almost $83 million in auction. The Cora Sun-Drop diamond is another large stone, coming in at 110.3 carats, and selling for a 2010 record of more than $10 million at auction. All diamonds, of course, do not fall into this extraordinary price category.