The Short Version
Diamonds are not among the rarest gemstones on Earth. While diamond formation requires specific geological conditions, diamonds are actually among the most common gemstones by volume — the International Gem Society calls them "likely the most common gem in nature." Numerous gemstones, including Red Beryl (1,000+ times rarer), Painite, Tanzanite, and Alexandrite, dramatically exceed diamonds in scarcity. The perception of diamond rarity was largely shaped by marketing, not geological reality.