This journal article is one of a series of excerpts from the author's unpublished book on gem materials. Amber: Insects caught in the hardening resin often produce swirls caused by struggling. Imitations include glass and plastic. Glass is denser, harder and colder to touch. Amber glows differently under ultraviolet lamps. Cut amber and copal resins break away in powdery splinters and chips and produce an aromatic smell when burnt. Jet, popular in Victorian England, has also been mined since Roman times, in Spain, France, the United States, Germany and Russia. A fossil wood allied to brown coal, it is derived from driftwood subjected to chemical action in stagnant water, then flattened by intense pressure. Jet melts and gives off smell of burning coal when pressed with a heated needle. Imitations include vulcanite, glass, obsidian, stained chalcedony ( black onyx ), albertite, bakelite, celluloid, casein, perspex and polystyrene. Main ivory (dentine) sources are the tusks from African and Asiatic elephants. Physical characteristics vary according to age and geographic location. Soft ivory withstands temperature differences better. Secondary sources are the tusks and teeth of walrus, cachelot (sperm whale) and narwhale.