Some properties of ± pure siliceous concretions such as flint (synonym: Feuerstein or chert (synonym; Hornstein), for example: density, alkali solubility, H20-absorption, specific surfaces, whethering resistance, solubility rates or sensibility to alkali reaction (when used as aggregates in concrete), are sufficiently explicable by a crystal-size model given in fig. 1, which may be valid for all siliceous concretions containing ± only quartz of different states, although it has been verified so far only for baltic glacial flint (investigations following below) and indirectly for chert from Ohio by KNELLER et al. (1968). If one takes for granted that all quartz crystals have a well ordered ‘core’ (brickwork pattern in fig. 1) with a ‘skin’ of always equal thickness with continuously growing structural disorder to the outside surface (dotted pattern in fig. 1) and subsequent lower density, then the density of siliceous concretions should increase with growing crystal sizes. This postulated correlation is proved in fig. 2 + 3, where the crystal sizes are characterized by specific surface data, measured for four different bulk density classes (fig. 2) and respective total densities (fig. 3) of V2 to. of baltic glacial flint material from 8 different deposits. The different densities are not due to various porosities (compare fig. 2 with fig. 3, bulk density versus total density), different silica modifications (microscopically and by X-rays only quartz has been observed) or different chemical composition (proved by chemical analyses).