The only opal mines in the world are those of Hungary. About four centuries ago, opals were obtained, in such abundance, from these mines, that upwards of three hundred persons were employed in them. They still produce opals, some of which are so valuable as to pass, in commerce, under the appellation of _oriental opals_, whilst others are so poor as to be of no value whatever to the jeweller. Opals are also found in other parts of Europe; and in the island of Sumatra and several parts of the East Indies. Few precious stones are more beautiful than opals. Their elegant play of colours, brilliant blue, green, red, and yellow, variously modified, has procured for them a distinguished rank among gems. Notwithstanding this, they are but ill suited to the purposes of jewellery, on account of their softness, their great frangibility, and their sometimes splitting on a sudden change of temperature. They are usually set without bottoms; but sometimes with a black bottom, and sometimes with a foil of red, blue, or gold colour. Their value is such that a fine oriental opal is considered worth about twice as much as an oriental sapphire of the same size. By the Turks they are so peculiarly esteemed, that a fine opal of moderate size has sometimes been sold at the price of a diamond. The esteem in which they were held among the ancient Romans was such, that Nonius, the Roman senator, is stated to have preferred banishment to parting with a favourite opal which Mark Antony was anxious to possess. In the abbey of St. Denys, near Paris, there was formerly a curious ancient opal which was green on the outside, and, when viewed against the light, exhibited a fine ruby colour: and in the Imperial Cabinet at Vienna, there are two pieces of opal, from the mines in Hungary, one of which is about five inches long, and 2½ inches broad; and the other the size and shape of a hen’s egg. Both these stones exhibit a very rich and splendid play of colours. In the purchasing of opals great caution is requisite, as fine glass pastes have not unfrequently been substituted for them, and sold at enormous prices. 103. HYDROPHANOUS OPAL, _or_ OCULUS MUNDI, _is a kind of opal, the distinguishing characteristic of which is, that it gradually becomes transparent, and exhibits a beautiful play of colour after being immersed in water_. It is either of a whitish brown, yellowish green, milky grey, or yellow colour, and opaque; and, when touched by the tongue, adheres to it. The name of oculus mundi has been given to these stones from an internal luminous spot, which changes its position according to the direction in which they are held to the light. The countries in which they are chiefly found are Hungary and Iceland.