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Cut and polished tanzanite stones are extremely rare — even more so than diamonds. Patrick AVENTURIER/Gamma-Rapho via Getty Images
If rarity boosts a gemstone's value, tanzanite, which is mined from just one source, could be prized for that fact alone. All the world's tanzanite is sourced from just 8 square miles (20 square kilometers) near Mount Kilimanjaro in Tanzania. By contrast, diamonds, which are considered rare, are found in more than 30 countries and on multiple continents.
Recognizable for its vibrant blue-violet color, tanzanite owes its place among today's favorite gemstones not only to its recent discovery, but also to a modern publicity campaign that catapulted the gem to fame.