Azulene is a molecule with 10 carbons and 8 hydrogens, consisting of a five-membered ring fused to a seven-membered ring. It is an isomer of naphthalene but its physical properties are quite different. Naphthalene is a white crystaline solid used in mothballs, whereas azulene, whose name is derived from the Spanish word azul, meaning "blue", is a dark blue crystaline solid used in many cosmetics.