It is often harvested by searching for greyish, ovoid lumps on the beach or floating in the ocean, barely beneath the surface. Anyone can be a professional ambergris harvester! The color can range from black to brown to yellow to grey and seems to be produced only by large males. Perhaps Captain Ahab was actually just smelling ambergris, which led him to his targets.
It's interesting to note that real ambergris often has squid beaks embedded inside. The theory that has surfaced is that the stomach of the sperm whales produces a substance to "wrap" the sharp objects and protect the whale from any internal damage. Makes sense to me - although only being produced by large males seems to dispute this theory at the outset. Perhaps I'm still focusing on the possibilities of finding squid beaks a bit intensely.
This particular photo shows a nice range of coloration you may see in ambergris, thanks to ambergris.fr :