Ambergris, the precious animalic substance that stabilizes and enhances fine perfumes and essences, is very rare in its natural form, therefore very expensive, since unfortunately its source has become more and more rare. The giant sperm whale, which likes to eat giant squid, mysteriously creates this material from the attempt to digest or perhaps coat its own digestive organs from the squid's sharp beak and other indigestible parts, and then horks the mass up into the ocean. It floats and the sun and salts in the seawater refine and naturally process it for a decade from black to gray to white, when it reaches its most fragrant and valuable incarnation. Washing up on shore, upon being found and gathered by the very lucky (like finding jewels on the beach) it is further processed and used in many perfumes. It is also used to flavor tea, and sometimes ice cream and candies. A speck stuck inside the lid of a teapot will impart the aroma for a year. Ingested, it it is believed to have almost a systemic, estrogen like effect; similarly maintaining youthfulness and fertility. Creed uses natural ambergris as a signature element in their base notes. Unfortunately I have never had the opportunity to try any in its unadulterated state, but I see that the Profumo site has some available in tincture form, and I am getting very tempted to obtain it from them soon. Luca Turin has some commentary about fooling around with it on the Profumo site and his readers contributed intriguing information on their experiences with this substance. It is known to have an almost magically enhancing, softening, blending effect on other perfume elements. During the Renaissance it was formed into beads and worn as fragrant jewelry, and carried in hand to ward off the noxious odors of the streets and illnesses. Luckily, it does not require animal sacrifice to obtain this substance, but it does require the continued existence of sperm whales and giant squid, two the of earth's largest animals, in enough numbers to make it possible for this material to be created naturally in large enough amounts to be useful. Apparently the imitations are not as wonderful as the original, and of course they can't have that special power of enhancement and fixation of other elements in perfume.
Here is a site that teaches how to identify it if you should be so fortunate as to find some on your travels and another with more information and lore.
Creed's Ambre Cannelle, Fleurs de Bulgarie, and Angelique Enscens, Ayala Moriel's Razala and Fairchild by Anya's Garden among so many others use it as an element.
Above, Fumée d'Ambre Gris - John Singer Sargent, 1880, his interpretation of inhaling and scenting a veil with ambergris incense from his time in Tangier...