Eye agates are most commonly found in agate nodules. ... Shortly after an initial layer of silica is deposited on the walls of the cavity, round droplets of silica gel form on the walls and begin to ... This origin on the inside walls of a cavity explains why eyes are most commonly found in the outer "husk" of an agate nodule.
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.
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.
Water-Line Agate (also called water-level or onyx) 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.