Mummy, unicorn's horn, and bezoars appealed to the imagination because of their unusual character, but even the most commonplace substances might develop supposedly medicinal virtues if they had unusual or gruesome associations.
Usnea was a substance of this nature. It was moss; not ordinary moss, but moss scraped from the skull of a criminal who had been hung in chains. Usnea was an official drug in the pharmacopeia until the nineteenth century; it was carried by all apothecary shops, and the first edition of the Encyclopedia Britannica devoted a section to its curative properties.
Usnea was present in the prescriptions of the best physicians over a period extending from the Middle Ages until well after the American Revolution.
Source: Howard W. Haggard, Devils, drugs, and doctors (1929).
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