Azulene is an organic compound and an isomer of naphthalene. Whereas naphthalene is colorless, azulene is dark blue. Its name is derived from the Spanish word azul, meaning "blue". Two terpenoids, vetivazulene (4,8-dimethyl-2-isopropylazulene) and guaiazulene (1,4-dimethyl-7-isopropylazulene), that feature the azulene skeleton are found in nature as constituents of pigments in mushrooms, guaiac wood oil, and some marine invertebrates.
