Modern assessments of the suitability of aggregates for concrete frequently include the petrographical examination of a single set of samples taken from stockpiles at a particular time. This study has shown that a structured petrographical study (SPS) carried out over a period of production gives a more reliable representation of the aggregate composition and can also be used to provide reassurance about the probable future uniformity of the material. Two flint-dominated sand and gravel sources in regular operation were selected: one sea-dredged and the other land-based. Sets of processed coarse and fine aggregate samples were taken monthly during a year of continuous exploitation and supply. The samples were each subjected to quantitative petrographical examination in accordance with the draft British Standard procedure. The results have been evaluated and indicate for these samples a higher degree of compositional uniformity over a period of production than might have been anticipated from a preliminary geological consideration of the source deposits. In both cases, the main constituents repetitively occurred, but the relative proportions varied within limits. In this single comparison of one sea-dredged source with one land-based source, the sea-dredged aggregate was marginally the less variable material. The practical application of SPS is illustrated with regard to the control of flint or chert content in aggregate combinations.