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 silicone 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.
Because of their natural beauty agates are used to make jewelry, bookends, wind chimes, and a host of other decorative items.
Industry uses agates chiefly to make ornaments such as pins, brooches or other types of jewellery, paper knives, inkstands, marbles and seals. Agate is also still used today for decorative displays, cabochons, beads, carvings and Intarsia art as well as face-polished and tumble-polished specimens of varying size and origin. Because of its hardness and ability to resist acids, agate is used to make mortars and pestles to crush and mix chemicals. Because of the high polish possible with agate it has been used for centuries for leather burnishing tools.
Fortification Agate
This is the most common and distinctive type of agate. In these agates, bands crystallized into concentric layers that more or less follow the shape of the cavity. The banding resembles an aerial view of a fort.
Water-Line Agate
During its formation, silica-rich solution either entered the cavity slowly laying one band at a time, excess water drained out leaving a small amount of silica to settle out into separate bands, or the vesicle filled under low pressure. In any case, the silica crystallization in water-level agates is controlled by gravity in one orientation, forming a stack up of parallel bands.
Shadow Agate
some agates exhibit a shadow effect in which there is what appears to be movement across the agate layers when the specimen is rocked back and forth (or your orientation to the specimen moves). This optical effect only occurs when there are alternating translucent and opaque bands. It is caused when light goes into the agate, but is not reflected out.
Tube Agate
Tube Agate During the formation of some agates, parallel projections of tiny hair-thin mineral rods grow first within the silica gel, around which later the chalcedony agate micro-crystals form. In some cases the inclusions remain, but sometimes they weather away leaving hollow tubes that subsequently fill in with silica.
Eye Agates
This mysterious agate characteristic is believed to be formed when most of the silica gel drains from the cavity, leaving only a droplet that beads up on the inside wall of the cavity. This droplet crystallizes into solid chalcedony eyes. In some cases, crystal growth will continue from this droplet in a manner similar to the formation of stalactites in caves. Later, the pocket fills in with the usual deposition of chalcedony microcrystals.
