There must be any number of self-respecting gemmologists out there on first-name terms with other diamonds, but for most of us the Koh-i-Noor is pretty well it. Most of what we think we know might be myth, guesswork or just plain wrong, and yet in spite of — or perhaps because of — that, the diamond which once adorned the Mughal empire’s Peacock Throne still retains all its old, ambiguous allure.