Agate

Agate is the term applied not to a distinct mineral species, but also to an aggregate of various forms of silica, chiefly Chalcedony. According to Theophrastus the agate was named from the river Achates, in Sicily, where these stones were first found. Most agates occur as nodules in eruptive rocks, and ancient lavas, where they represent cavities originally produced by the disengagement of a vapor in the molten mass, and then since filled, wholly and partially, by siliceous matter deposited in regular layers upon the walls. Such agates, when cut transversely, exhibit a succession of parallel lines, often of extreme tenacity, giving a banded appearance to the section; whence such stones are known as banded agate, rebind agate and striped agate.

In the formation for an ordinary agate, it is probable that waters containing silica in solution -- derived, perhaps, from the decomposition of few of the silicates in the lava itself -- percolated through the rock, and then deposited a siliceous coating on the interior of the vapor-vesicles. Variations in the character of the solution, and in the conditions of deposit, may have caused corresponding variation in the successive layers, so that bands of chalcedony often alternate with layers of the crystalline quartz, Several vapor-vesicles could unite while the rock is viscous, and hence form a large cavity which could become the home of an agate of exceptional size; thus a Brazilian geode, lined with amethyst, of the weight of 40 tons, was exhibited at the Dusseldorf Exhibition during 1902.

Many agates are hollow, and since deposition had not proceeded far enough to fill the cavity, and in such cases the last deposit commonly consists of quartz, and then very often amethyst, having the apices of the crystals directed towards the free space, so as to form a crystal-lined cavity and geode.

A Mexican agate, showing only a single eye, had received the name of "cyclops." Included matter of the green colour, like fragments of "green earth," embedded in the chalcedony and also disposed in filaments and other forms suggestive of vegetable growth, gives rise to moss agate.