Greeks named diamonds, April’s birthstone, adamas, meaning “unconquerable”—a fitting moniker for the hardest substance found in nature. Mined most famously in South Africa, but also in Australia, Canada, Russia, India, Brazil, and even Borneo, diamonds are the oldest of all gemstones. (Most are more than 100 million years old.) While the gems are commonly thought of as colorless, they are available in a veritable rainbow of colors, from brown to ultra-rare blue. Greeks often thought of them as teardrops from the gods, while Romans believed they came from the falling stars that tipped the arrow of Eros, the god of love. For Hindus, diamonds were the lightning that armed the hand of the god Indra.