The probable field performance of concretionary gravel aggregates is difficult to predict in the tropics, mainly due to the lack of understanding of the factors that control their engineering properties. The mineralogy and fabric of commonly used laterite gravel aggregates in Nigeria were studied to assess their influence on the engineering properties. Thin- section studies revealed two characteristic features which greatly influence the physico-mechanical properties: (a) abundant voids and fissures of different sizes and (b) varying degree of iron oxide impregnation. The variations in these characteristics are due to differences in the degree of lateritization. Strong and well developed gravels and pisolites are composed of 65–95 0oethite/hematite and are dominated by small voids. Weak and immature laterite gravels contain much larger pores and greater amounts of quartz and kaolinite. The quartz grains show extensive cracking which futther reduces the mechanical strength of these laterite gravels. The void/fracture-dominated fabric of laterite gravel aggregates is mainly responsible for their unusually high absorption/porosity and relatively low strength. In some laterite gravels, the strength loss on wetting is mainly due to weak cementation and the presence of interstitial clays.