Seam agate, (sheet-like slabs of variegated or multi-colored cryptocrystalline silica) is a common component of the Tallahatta Formation throughout much of southwestern Alabama. Amateur geologists have long collected this material to produce attractive polished slabs and they have come to call it Alabama Agate. They have also used the name Alabama Blue Agate for specimens characterized by a dark grey-blue tint. From a geological point of view, the term "agate" is usually applied to the chalcedony variety of cryptocrystalline silica, particularly when it is semi-transparent. Much of the cryptocrystalline silica in the Tallahatta Formation satisfies the definition of "agate", but under the petrographic microscope, most does not. Chalcedony is characterized by an intricate fan like appearance when viewed under crossed Nichols (see petrology section). The majority of the cryptocrystalline silica in the Tallahatta Formation is better termed chert as it clearly formed through the replacement of an earlier rock type (probably limestone). In order to apply a standard geological nomenclature to the lithic materials in Alabama, we advocate naming all cryptocrystalline seams within the Tallahatta Formation "Tallahatta Chert". This name recognizes the cryptocrystalline nature of the material, distinguishes its source formation, and isolates it from the more abundant Tallahatta Sandstone that occurs with it throughout southwestern Alabama