Agate

Agate
Banded agate (agate-like onyx); the specimen is 2.5 cm (0.98 in) wide
General
Category	Quartz variety
Formula (repeating unit)	SiO2 silicon dioxide
Crystal system	Rhombohedral Microcrystalline
Identification
Color	White to grey, light blue, orange to red, black. banded
Crystal habit	silica
Cleavage	None
Fracture	Conchoidal with very sharp edges.
Mohs scale hardness	6.5–7
Luster	Waxy
Streak	White
Diaphaneity	Translucent
Specific gravity	2.58–2.64
Refractive index	1.530–1.540
Birefringence	up to +0.004 (B-G)
Pleochroism	Absent

Agate /ˈæɡət/ is a 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.

Agate is one of the most common materials used in the art of hardstone carving, and has been recovered at a number of ancient sites, indicating its widespread use in the ancient world; for example, archaeological recovery at the Knossos site on Crete illustrates its role in Bronze Age Minoan culture.

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 , giving a banded appearance to the section. Such stones are known as banded agate, riband agate and striped agate.