Alchemy - The fall of Nostoc, the heavenly dew. Quadrafoil on the west front of Amiens Cathedral, France. 14th century. Nostoc is a genus of fresh water cyanobacteria that forms spherical colonies composed of filaments of moniliform cells in a gelatinous sheath. When on the ground, a Nostoc colony is ordinarily not seen; but after a rain it swells up into a conspicuous jellylike mass, which was once thought to have fallen from the sky, whence the popular names, fallen star and star jelly. It is also called witches' butter. ?006 TopFoto / Fortean