A mineral whose basis is calcedony, blended with variable proportions of jasper, amethyst, quartz, opal, heliotrope, and carnelian. Ribbon agate consists of alterate and parallel layers of calcedony with jasper, or quartz, or amethyst. The most beautiful comes from Liberia and Saxony; it occurs in porphyry and gneiss. Brecciated agate is of Saxon origin; it has a base of amethyst, containing fragments of ribbon agate, constituting the beautiful variety. Fortification agate, found in Scotland and on the Rhine, is in nodules of various shapes, imbedded in amygdaloid. On cutting it across, and polishing it, the interior zig-zag parallel lines bear a considerable resemblance to the plan of a modern fortification. In the very centre, quartz and amethyst are seen in a splintery mass, surrounded by the jasper and calcedony. Mocha stone, from Mocha, in Arabia, where it is chiefly found, is translucent calcedony, containing dark outlines of arborization, like vegetable filaments. Moss agate, so called from its ramifications of a vegetable form, is a calcedony, variously coloured, and occasionally traversed with irregular veins of red jasper.

An onyx agate set in a ring belonging to the earl of Powis contains the chrysalis of a moth.

Agate is found in most countries, chiefly in trap rocks and serpentine. The oriental agate is almost transparent, and of a vitreous appearance, The occidental is of various colours, and often veined with quartz or jasper. Agates are most prized when the internal figure nearly resembles some animal or plant. Agates are artificially coloured by immersion in metallic solutions. They are extensively used in Paris for making cups, rings, seals, handles for knives and forks, sword-hilts, beads, smelling-bottles, snuff-boxes, etc. At Oberstein, on the Rhine, where the stones are abundant, they are cut and polished on a considerable scale, and at a very moderate price. The surface to be polished is first coarsely ground by large millstones of a hard reddish sandstone, moved by water. The polish is afterwards given on a wheel of.soft wood, moistened and imbued with a fine powder of hard red tripoli, found in the neighbourhood. Antiquaries use the term agate to denote a stone of the kind engraved by art. In this sense agates make a species of antique gems, in the workmanship of which we find eminent proofs of the great skill and dexterity of the sculptor.

Several agates of exquisite beauty are preserved in the cabinets of the curious.