In the jewellery world, diamond quality is assessed using the '4C's' - carat weight, colour, clarity and cut. The first three are determined by geology; only cut is controlled by humans.
Diamonds are formed in the Earth between one billion and three billion years ago. The recipe goes something like this: Bury carbon-containing minerals 160 kilometres underground, heat them at 1200°C, squeeze under extreme pressure(about 7 gigapascals), then quickly rush this mixture(via volcanic eruption) to the Earth's surface to cool.
Rarity and beauty make diamonds the perfect symbol to represent true love, magnificent achievements and important milestones.