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Agate

Quartz and Silica Minerals Gallery

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Agate is a rock (and a gemstone) composed chiefly of chalcedony. This is a particularly refined specimen from Indonesia. (more below)
A gemstone specimen
Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)
Agate is the same kind of rock as chert, but in a much purer, more transparent form. It consists of amorphous or cryptocrystalline silica, the mineral chalcedony. Agate forms from solutions of silica at relatively shallow depths and low temperatures, and is exquisitely sensitive to the physical and chemical conditions around it. It is commonly associated with the silica mineral opal. Fossilization, soil formation, and alteration of existing rock can all create agate.

Agate occurs in infinite variety and is a favorite material among lapidaries. Its fluid forms lend themselves to attractive cabochons and similar flat or rounded jewelry formats.

Agate may have several different names, including carnelian, catseye and many fanciful names suggested by the shapes and colors of a particular occurrence.

This stone, magnified several times, displays cracks that extend only a few millimeters from the surface. They are completely healed and do not affect the stone's strength. For a larger specimen, see the agatized tree-trunk in the Fossil Wood Gallery.

For deep geologic information on agates, including hundreds of pictures, visit the Agate Resources page from the University of Nebraska. Agate is the state rock or state gemstone of Florida, Kentucky, Louisiana, Maryland, Minnesota, Montana, Nebraska and North Dakota.

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