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._