What are the rarest gemstones on Earth?

You're right that generally speaking diamonds aren't the rarest gemstones on earth. Small, clear diamonds are even relatively common. The larger the diamond, however, the rarer it is, and deeply colored diamonds, known as "fancies" or fany colored diamonds, are exceedingly scarce. Red diamonds are considered the rarest of the colored diamonds, with only some 35 known to exist worldwide, almost all of which weigh less than 1/2 carat! Some famous red diamonds are the Red Sheild (5.11 carats), the Moussaieff Red and the De Young Red diamond. Untreated red diamonds go for from about $800,000 to $2 million per carat. Another very rare gem is alexandrite. Alexandrite was discovered for the first time only about 150 years ago near the Ural mountains. Besides being so rare, the gemstone actually changes color in different lighting, from blue-green to purplish red. It is found mostly in Russia, but also in Brazil, Madagascar, Zimbabwe, Burma and India. The price per carat of a high-quality alexandrite is about $5000-$10000. Compare this to diamond price per carat, which is about $1000-$2000. Serendibite is also a rare, cyan-colored gemstone that is found in Sri Lanka.

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