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What is agate?

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frequent semiprecious silica mineral, a wide range of chalcedony (q.v.) that occurs in bands of various color and transparency. Agate is essentially quartz, and its physical properties are usually those of that mineral.

Agate is discovered all through the world. Within the United States it's produced in a number of western states; Oregon, Washington, Idaho, and Montana are the chief sources of gemstones. Most agates happen in cavities in eruptive rocks or historical lavas. These agates have a banded construction, successive layers being roughly parallel to the perimeters of the cavity. It is possible that they have been formed as follows. Throughout cooling of the lava, steam and other gases kind bubbles. The bubbles overtaken by solidification are frozen in, forming cavities. Long after the rock has solidified, water-carrying silica (q.v.) in solution, probably as alkali silicate-penetrates into the bubble and coagulates to a silica gel. Soluble elements of the iron-bearing rock diffuse into the silica gel and produce the regular layers of iron hydroxide. Lastly the whole mass gradually hardens with lack of water and crystallization of much of the silica as quartz or chert. Throughout crystallization, the coloured bands usually are not disturbed. Sorts of agate are characterised by peculiarities within the shape and color of the bands, which are seen in sections cut at right angles to the layers. In riband agate the bands seem as straight traces in cross section. Such agate, with white bands alternating with bands of black, brown, or crimson, is known as onyx (q.v.). A hoop or eye agate has concentric circular bands of different colours. A range having included matter of a green colour, embedded in the agate and disposed in filaments and different types suggestive of vegetable progress, is called moss agate (q.v.).

The agate-working industry grew up centuries ago in the Idar-Oberstein district of Germany, where agates have been abundant. After 1900 most agates have been supplied by Brazil and Uruguay. Most commercial agate is artificially stained in order that stones unattractive due to their uninteresting-grey tints grow to be beneficial for ornamental purposes.