The most beautiful agates which our island produces are known by the name of Scots Pebbles. These are found in various parts of Scotland, but principally on the sea-shore, in the neighbourhood of Dunbar. Agate pebbles are found on several of the English shores, as those of Suffolk, Dorset, Scotland, Wales, and Ireland; and sometimes even in gravel pits. Many of them will bear cutting and polishing as well as the best agates of foreign countries.
Agates are occasionally seen to be figured in very singular manner; but this, in some instances at least, is suspected to be the work of art. One is mentioned in the church of St. Mark, at Venice, which had the representation of a king’s head surmounted by a diadem. On another, was represented a man in the attitude of running. But the most remarkable of all seems to have been one which contained a representation of the nine Muses, with Apollo in the midst of them!
It must be remarked that agate is not, as some mineralogists imagine, a simple mineral, but that it is composed of various species of the quartz family, intimately blended together. It consists chiefly of calcedony ([91]), with flint, hornstone, carnelian ([93]), jasper ([96]), cacholong ([105]), amethyst ([79]), and quartz ([76]). Of these minerals sometimes only two, and sometimes three or more, occur in the same agate. Its varieties, consequently, are extremely numerous.
101. Mochoa Stone is a kind of agate, which has on its surface the resemblance of moss; and this so nearly approaching a natural appearance, that some persons have actually supposed it to be occasioned by a condensation of moss into stone. Its name is derived from mocks, the German word for moss.
These stones are used for several ornamental purposes; and are not unfrequently imitated, by spreading a solution of copper in nitric acid or aqua fortis ([30]) over the surface of a plain agate, and then setting a small iron nail on its head in the middle. The acid unites with the iron, and deposits the copper in beautiful ramifications from the centre. The nail must then be removed, and the surface carefully washed by dipping the stone into warm water. Afterwards, on the application of a moderate heat, the copper becomes black. As, however, the deposition is merely superficial, it requires to be covered with glass, to preserve it from injury.
102. OPALS are a semi-transparent kind of stones, which have a milky cast, and, when held betwixt the eye and the light, exhibit a changeable appearance of colour.
They are always found in a shapeless or massive state, are brittle, and considerably less hard than most other precious stones.
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.