Agate // is a cryptocrystalline variety of silica, chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks and can be common in certain metamorphic rocks.The stone was given its name by Theophrastus, a Greek philosopher and naturalist, who discovered the stone along the shore line of the river Achates (Greek: Ἀχάτης) in present-day Sicily, sometime between the 4th and 3rd centuries BC.[ Colorful agates and other chalcedonies were obtained over 3,000 years ago from the Achates River, now called Dirillo.
Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles
in the original molten mass, which were then filled, wholly or
partially, by siliceous matter deposited in regular layers upon the
walls. Agate has also been known to fill veins or cracks in volcanic or
altered rock underlain by granitic intrusive masses. 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 vesicles.
Variations in the character of the solution or in the conditions of
deposition may cause a 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 still 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 and weighing 35 tons was exhibited at the Düsseldorf Exhibition of 1902.