Those round calcareous[757] excrescences found both in the bodies and shells, especially on the nacreous coat, of several kinds of shell-fish[758], have been much used as ornaments since the earliest ages[759]. The beautiful play of colours exhibited on their surface has raised them to a high value[760]; and this they have always retained on account of their scarcity and the expense arising from the laborious manner in which they are collected[761]. By the increase of luxury among the European nations, the use of pearls has become more common; and even in Pliny’s time they were worn by the wives of the inferior public officers, in order that they might vie in the costliness of their dress with ladies of the first rank. It is probable, therefore, that methods were early invented to occasion or hasten the formation of pearls; and as at present those who cannot afford to purchase gold, jewels, and porcelain, use in their stead pinchbeck, artificial gems, and stone-ware, so methods were fallen upon to make artificial pearls. The art of forcing shell-fish to produce pearls was known, in the first centuries of the christian æra to the inhabitants of the coasts of the Red-sea, as we are told by the philosopher Apollonius, who thought that circumstance worthy of particular notice. The Indians dived into the sea, after they had rendered it calm and more transparent by pouring oil into it. They then enticed the fish by means of some bait to open their shells; and having pricked them with a sharp pointed instrument, received the liquor that flowed from them in small holes made in an iron vessel, in which they hardened into real pearls[762]. Olearius says that this account is to be found in no other author: but it has at least been copied by Tzetzes[763].