Folk used to think it was whale vomit, dragon’s spittle, a mushroom, tree sap, a submarine pear, or something meteoritic. Charles II liked to put it on his eggs, the Chinese swore by it as a medicine, and Casanova used it to pep up his chocolate mousse. It can retain its peculiar aroma – tobacco? violets? cow pat? – for 300 years, and for many centuries has been prized by apothecaries, perfume makers and aficionados of rarity. These days it’s valued by traders at $1,000 a pound, but you have to be prepared to pick up a lot of noisome detritus if you want to be a serious collector. Welcome to the world according to ambergris.