Agate

Silicon Dioxide SiO₂

Fire agate

Agate is the name given to a group of silicate minerals that are made up primarily of chalcedony. Chalcedony is a member of the quartz family of minerals. Like quartz chalcedony is silicon dioxide with a chemical formula of SiO₂. The main difference between quartz and chalcedony is in the size of the individuals crystals. Chalcedony is termed microcrystaline meaning that individual crystals are too small to be seen by the naked eye.

Agates are formed primarily in cavities of volcanic rock. Over time silica rich water seeping through the rock deposits minerals on the inside of these cavities. Gradually layers of silicate material build up eventually filling the cavity completely. The layers often have subtle differences in mineral content giving the agate a banded appearance.

When the cavity is only partially filled the inner layer usually shows a crystalline structure. This is often quartz, amethyst, or related mineral. This hollow rock is called a <b>geode</b>. While most often small geodes can be huge occasionally big enough to walk around in!

When the cavity completely fills the rock is no longer called a geode. It is called a thunder egg. Thunder eggs can be as varied as geodes. Oregon is famous for collecting thunder eggs.

agate, Utah

There are many kinds of agate owing to slight changes in mineral content. These slight changes affect the color and the patterns within the rock. Descriptive and colorful names have been given to these unique varieties. Moss agate, carnelian agate, Botswana agate, blue lace agate, and Mexican crazy-lace agate are just a few of these individual types.

Because of their natural beauty agates are used to make jewelry, bookends, wind chimes, and a host of other decorative items.

Mineral Properties

Chemical formula: Silicon dioxide SiO₂
Color(s): many colors and patterns
Streak: white
Luster: vitreous, glassy
Transparency: transparent to translucent
Crystal system: triagonal
Specific Gravity: 2.6
Hardness (Mohs): 7
Cleavage: none
Fracture: conchoidal
Uses: jewelry, bookends, wind chimes, and other decorative items.
Location: widespread occurance

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