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.
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.
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 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.
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.
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