Purple banding is evident in this geode agate.
Agates are minerals that have banding and multicolored attributes. Agates belong to the quartz family. Layers of agates form as chalcedony in a host rock. Silicon dioxide is the chalcedony material. Bands on many agate stones wrap around the stone center like tree rings. Typically, raw agates are dull and bland in appearance, and they must be polished for their more attractive qualities to surface.
Fortification Agate
The most common type of agate is a fortification agate. Bands crystallize into concentric layers that generally follow the shape of the cavity. The banding resembles an aerial topography map.
Geode Agate
Geode agates have is a hollow center within a crystalline outer fill and agate banding on the outside. The inner layers can form into different types of silica mineral crystals.
Water-Line Agate
These agates form by silica-rich solution entering the cavity slowly, laying one band at a time, or the cavity filled under low pressure. The silica crystallization in water-line agates is controlled by gravity, forming a stack of parallel bands.
Tube Agate
During the formation of tube agates, parallel projections of tiny mineral rods grow first within silica gel, around which agate micro-crystals form. Sometimes the inclusions weather away, leaving hollow tubes that fill with silica.
Moss Agate
These agates have mineral inclusions that resemble trees or plants. Typically, moss agate inclusions are made of iron or manganese oxide. In some cases, the inclusions inhibit agate banding, allowing the mineral clusters to grow freely.
Shadow Agate
Certain agates display a shadow effect that appears to exhibit movement across the agate layers when the specimen is moved in the field of vision. Alternating opaque and translucent bands must be present for this effect to occur. The cause is light entering the agate that is not reflected out.
Eye Agate
The theory of eye agate formation is that silica gel drains from the cavity, leaving a droplet that form a bead on the inside cavity wall. This droplet crystallizes into a structure that resembles eyes. Occasionally, growth continues from this droplet in a similar manner to cave stalactite formation.
Seam Agate
This agate forms in cracks within host rock, instead of in rounder pockets. The bands form in rows that fill in the crack or seam.
Plume Agate
Plume agates had formations on their outside layer that formed before the depositing of chalcedony bands. These formations are filament mineral inclusions that resemble feathers or ferns.
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