3 facts
While diamonds are exceptionally hard, materials like Wurtzite Boron Nitride and Lonsdaleite have been theoretically or experimentally shown to be harder. These are rare and don't occur naturally in large quantities.
While natural diamonds are precious, they are abundant beneath the Earth's surface. The perception of their rarity is largely due to controlled supply by mining companies, which limits the number released into the market to maintain high prices.
The Earth's core contains an estimated 1.6 quadrillion tons of gold, enough to cover the planet's entire surface with a layer nearly half a meter thick. However, it's largely inaccessible.