the help of the architect Decrianus and 24 elephants, to a position next to the Flavian Amphitheatre (now called the Colosseum). Pliny gives a special place to iron, distinguishing the hardness of steel from what is now called wrought iron, a softer grade. He is scathing about the use of iron in warfare. Mineralogy In the last two books of the work (Books XXXVI and XXXVII), Pliny describes many different minerals and gemstones, building on works by Theophrastus and other authors. The topic concentrates on the most valuable gemstones, and he criticises the obsession with luxury products such as engraved gems and hardstone carvings. He provides a thorough discussion of the properties of fluorspar, noting that it is carved into vases and other decorative objects. The account of magnetism includes the myth of Magnes the shepherd. Pliny moves into crystallography and mineralogy, describing the octahedral shape of the diamond and recording that diamond dust is used by gem engravers to cut and polish other gems, owing to its great hardness. He states that rock crystal is valuable for its transparency and hardness, and can be carved into vessels and implements. He relates the story of a woman who owned a ladle made of the mineral, paying the sum of 150,000 sesterces for the item. Nero deliberately broke two crystal cups when he realised that he was about to be deposed, so denying their use to anyone else. Pliny returns to the problem of fraud and the detection of false gems using several tests, including the scratch test, where counterfeit gems can be marked by a steel file, and genuine ones not. Perhaps it refers to glass imitations of jewellery gemstones. He refers to using one hard mineral to scratch another, presaging the Mohs hardness scale. Diamond sits at the top of the series because, Pliny says, it will scratch all other minerals. Art history Pliny's chapters on Roman and Greek art are especially valuable because his work is virtually the only available classical source of information on the subject. In the history of art, the original Greek authorities are Duris of Samos, Xenocrates of Sicyon, and Antigonus of Carystus. The anecdotic element has been ascribed to Duris (XXXIV:61); the notices of the successive developments of art and the list of workers in bronze and painters to Xenocrates; and a large amount of miscellaneous information to Antigonus. Both Xenocrates and Antigonus