Most agates occur as nodules in eruptive rocks, or ancient lavas, where they represent cavities originally produced by the disengagement of vapour in the molten mass, and since filled, wholly or 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 tenuity, giving a banded appearance to the section, whence such stones are known as banded agate, riband agate and striped agate.
In the formation of an ordinary agate, it is probable that waters containing silica in solution -- derived, perhaps, from the decomposition of some of the silicates in the lava itself -- percolated through the rock, and deposited a siliceous coating on the interior of the vapour-vesicles. Variations in the character of the solution, or in the conditions of deposit, may have caused corresponding variation in the successive layers, so that bands of chalcedony often alternate with layers of crystalline quartz, Several vapour-vesicles may unite while the rock is viscous, and thus form a large cavity which may become the home of an agate of exceptional size; thus a Brazilian geode, lined with amethyst, of the weight of 35 tons, was exhibited at the Dusseldorf Exhibition of 1902.
The first deposit on the wall of a cavity, forming the "skin" of the agate, is generally a dark greenish mineral substance, like celadonite, delessite or "green earth," which are rich in iron, derived probably from the decomposition of the augite in the mother-rock. This green silicate may give rise by alteration to a brown oxide of iron (limonite), producing a rusty appearance on the outside of the agate-nodule. The outer surface of an agate, freed from its matrix, is often pitted and rough, apparently in consequence of the removal of the original coating. The first layer spread over the wall of the cavity has been called the "priming," and upon this basis zeolitic minerals may be deposited.
Many agates are hollow, since deposition has not proceeded far enough to fill the cavity, and in such cases the last deposit commonly consists of quartz, often amethyst, having the apices of the crystals directed towards the free space, so as to form a crystal-lined cavity or geode.
A Mexican agate, showing only a single eye, has received the name of "cyclops." Included matter of a green colour, like fragments of "green earth," embedded in the chalcedony and disposed in filaments and other forms suggestive of vegetable growth, gives rise to moss agate.
Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum, due to the extreme delicacy of the successive bands, whence they are termed rainbow agates.
On the disintegration of the matrix in which the agates are embedded, they are set free, and, being by their siliceous nature extremely resistant to the action of air and water, remain as nodules in the soil and gravel, or become rolled as pebbles in the streams.
See also: list of minerals