Whale, sage, and the smell of skin that has been in the sun
For most of perfumery's history the smell called ambergris came from an actual substance: a waxy lump formed in the gut of a sperm whale, expelled, and left to drift for years until the sea and the sun turned it from something foul into something extraordinary. Washed up and aged, it smelled of warm skin, salt, tobacco and old paper at once, and it fixed everything it touched. It was also rare, expensive, and ethically impossible once the whale trade ended.
The chemistry of why it smelled that way was worked out in the first half of the twentieth century, largely at Firmenich in Geneva, where Leopold Ruzicka and later Max Stoll and their colleagues isolated the molecules responsible. The one that mattered most was a small, rigid, ether-bridged ring. In 1950 Firmenich found a way to make it without a whale.
The sage
The starting point was sclareol, a compound found in clary sage, a herb grown for its oil in the south of France and in Russia. Sclareol is cheap and has almost the right skeleton; a few steps of oxidation and rearrangement close it into the ambergris molecule. Firmenich sold the result as Ambrox, and the name became so generic that "Ambroxan" (a Henkel, later Symrise, trade name for the same molecule) is now used by perfumers for any of the half-dozen brands. The molecule is the same; the price, purity and isomer ratio vary.
What it does
Smelled neat it is faint, dry, faintly mineral, a little sweet, with something of a clean shirt and something of a beach. It barely projects on its own, which misled early users into overdosing it. Its gift is persistence: it stays on skin and on fabric for a day and a half, and it lifts everything layered on top of it so that a whole formula seems to last longer. Modern "ambery" masculine perfumes, the ones that smell like a well-dressed person rather than like a flower or a wood, usually rest on a large dose of it.
The story goes
The version told in perfumery circles is that the 1950 synthesis was an act of scientific curiosity that nobody expected to be commercial, because ambergris was still legal and nobody imagined a synthetic could replace it. Whether or not the chemists saw it coming, within a generation the synthetic had become the standard and the original had become a curiosity. It is one of the few cases where the copy is, by almost every practical measure, better than the thing it copies.
Open Ambroxan on your bench Its catalog page What replaces it
Sources: firmenich.com · Ohloff, G. (1994). Scent and Fragrances: The Fascination of Odors and Their Chemical Perspectives. Springer.