Ben Shattuck reviews Christopher Kemp's Floating Gold: A Natural (& Unnatural) History of Ambergris:
People have used ambergris (‘gray amber,’ French) for a long time — Moctezumaadded it to his tobacco, Casanova to his chocolate mousse, England’s King Charles II to his eggs; 17th-century French physicians used it to cure rabies, Florida’s American Indians as an antidote for fish poison, and today, companies like Chanel and Guerlain as fixative in their most expensive perfumes.
So what does it smell like?
Here is a solid lump of whale feces, weathered down—oxidized by salt water, degraded by sunlight, and eroded by waves — from the tarry mass to something that smells, depending on the piece and whom you’re talking to, like musk, violets, fresh-hewn wood, tobacco, dirt, Brazil nut, fern-copse, damp woods, new-mown hay, seaweed in the sun, the wood of old churches, or pretty much any other sweet-but-earthy scent. Borne in whale guts to be crushed and dabbed on the wrists and necks of the elite.
Synthetic alternatives have been discovered but Kemp isn't convinced:
Scientists may be able to produce compounds that can mimic the fixative properties of ambergris, but the odor of the ambergris is itself indescribable, let alone replicable.