A pseudomorph is something that takes the form of something else. Pseudomorphs in agate are quite rare but do occur in nodular agates from various locations, usually as a calcite or aragonite replacement. The Coyamito agate deposit in Chihuahua Mexico produces more pseudomorphs than any other agate location.

The formational process goes something like this. First crystals grow in the gas cavity left in the volcanic andesite rock (these crystals are believed to be aragonite in the case of the Coyamito Agate). The agate then starts forming, coating the crystals and the inside of the cavity. The more agate that forms the thicker the coating on the crystals. This process can continue until the gas cavity is filled or, more frequently stops, leaving a hollow portion in the nodule. Like all nodular agates, quartz or agate may form changing from one to the other.

As the agate layer forms over the crystal, the shape of the resulting formation gets smoother, until it becomes roughly spherical in cross section. A solid agate cut across such a formation would show a concentric banded pattern almost spherical on the outside and hexagonal toward its center (hexagonal at the point of first coating the crystal).

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