Agate, universally popular with infinitely variable appearance and color, contains a variety of silica polymorphs, amorphous and nanocrystalline silica, chalcedony and quartz. It is characterized by distinctive banded texture, and usually displays special rhythms with associated repetitive textures and compositions at different scales. Very often agate is simply defined as "banded chalcedony", and for most practical purposes this should be sufficient. However, microstructural and spectroscopic observations reveal that most agates, depending on their genesis, are mixed from fibrous chalcedony, wall-banded chalcedony, horizontally-banded chalcedony, microcrystalline quartz and euhedral crystalline quartz. X-ray diffraction and electron backscatter diffraction (EBSD) analyses indicate that most nanocrystalline and microcrystalline quartz in agate tends to grow with a-axes perpendicular to the growth substrate, typical of length-fast chalcedony. Fourier transform infrared spectroscopy (FTIR) analysis reveals some molecular water and silanol group water in agate, with lower total water content in crystalline silica than in amorphous silica. Although agate is abundant worldwide, controversy surrounds the genesis of its characteristic banded textures and the consensus has not been achieved. A representative hypothesis suggests that the bands in agate are either from precipitation from siliceous hydrothermal influxes or by in situ crystallization of a silica gel. However, most of the recent literature indicates a viable model for the growth of chalcedony in agate based on diagenetic cycles reflected in the degree of crystallinity, crystallographic orientation and water content. Quantitative information on microstructural observations and micro-scale analyses with petrological experimental results will improve our understanding of the origin and formation processes of agate.