The sequences of diagenetic changes of polymorphic silica modifications in weathering silcretes from volcanic rocks of Central and Eastern Slovakia was studied by comprehensive methods. It was found that these changes follow a similar way to that described for the diagenesis of biogenic opals, that is from opal (opal-A) through opal-CT-chalcedony-secondary quartz with gradual changes of silica crystallinity. In the formation of the different weathering silcrete types direct silica precipitation participated together with processes of alternate dissolution-reprecipitation and recrystallization and gravity transport. Direct precipitation of silica from colloidal solutions trough a gel stage is documented by the presence of colloidal fabric and features. "Pseudo-brecciated" textures of silcrete is evidence of gravity transport of the (aged) gel fragments. The progressive change in the crystallinity of secondary silica forms is usually a complex process which is connected with changes in the degree of saturation of weathering solution to the individual polymorphic silica phases and with the presence of some others components which may accelerate these changes. The weathering silcretes in the studied volcanic rocks are relict in nature. They originated in conditions characteristic of intensive (hydrolytic) weathering. Silcretes are juxtaposed (superimposed) on iron-rich illuviated kaolinitic clays or secondary Fe-, Mn- and Ti-oxides. They are preserved in the fissures and cavities of weathered rocks as erosion remnants of the paleoweathering crusts. Therefore these silcretes can serve as paleoweathering indicators.