Bergamot is a fragrant, sour, yellow-green citrus the size of a small orange, grown almost entirely along a narrow strip of the Calabrian coast in the far south of Italy. Its flesh is far too sour and bitter to eat, but the rind yields an intensely perfumed essential oil — the scent that flavours Earl Grey tea, the heart of classic eau de cologne, and a backbone of the modern perfumer's palette.