Agate

Agate, named from the river Achates in Sicily. where it was found, a form of quartz or silica. It consists mainly of the chalcedonic or non-crystalline variety, but contains layers of crystalline quartz. It occurs in rounded nodules in amygdaloid basaltic rocks, especially at Oberstein and Idar, on the Nahe, in Germany; in Uruguay, in New South Wales, in Scotland, and elsewhere. Lapidaries often know the stone as "Scotch pebble," but the chief factories, those in Germany, now derive their main supply from Uruguay, via Brazil. The nodules seem to have originated as bubbles, or infiltrations of gas-cavities, in the rock when fused, every gradation being traceable from the hollow "geode" or "potato-stone" with a mere lining of quartz-crystals to the perfectly filled agate. The various layers are of different tints, mostly of gray, but, varying in porosity, are artificially tinted at Oberstein to almost every colour, by boiling in metallic salts. If in regular concentric bands the agate is termed onyx; if in bands with an angular, zig-zag, or bastion-like outline, fortification-agate; whilst the subsequent infiltration of colouring-matters along fissures has produced the forms known as moss-agates and pagoda-stones. Fracture and re-infiltration have produced the ruin-agate. Agates are often found in river-gravels, having been liberated by the weathering of the rock containing them. By the ancients agate was chiefly valued as a material for carving cameos and intaglios, a layer of one colour being cut away so as to reveal another differently tinted. In addition to its use for ornamental purposes, seals, beads, rings, etc., agate is employed for metallurgical pestles and mortars.