Agate (Part 1)
Agate:
Agate is a term applied not to a distinct mineral species, but to an aggregate of various forms of silica, chiefly chalcedony.
According to Theophrastus, the agate (achates) was named from the river Achates, now the Drillo, in Sicily, where the stone was first found
Formation and characteristics:
Most agates occur as nodules in volcanic rocks or ancient lavas where they represent cavities originally produced by the disengagement of volatiles in the molten mass which were then 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. 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 cause 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 Braziliangeode lined with amethyst and weighing 35 tons was exhibited at the Dusseldorf Exhibition of 1902.
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[Source: Beads and Beading Blog]