The basic underlying assumption of grain-shape studies of sediment sources is that the shapes of quartz grains from a given source reflect the genesis or lithology of that source and are distinct from the shapes of quartz grains from other sources. However, this critical assumption has never been tested and thus is a source of doubt about the results of past grain-shape studies. Therefore, a study of the original shapes of quartz sand grains that are released from crystalline and sedimentary rocks was conducted. The study shows that this critical assumption is true for quartz grains that are released from crystalline rocks, wind-transported sedimentary rocks, and quartz-cemented sedimentary rocks. These quartz grains were subjectd to varied mechanical and chemical processes during the genesis of these sources, and these processes have imprinted the grains with unique, source-indicative shapes. Quartz grains from crystalline rocks have highly nonspherical, angular outlines and numerous crystalline nodes, grain embayments, and fractures which are the products of crystallization and deformation at elevated temperatures and pressures. Quartz grains from wind-transported sedimentary rocks have highly rounded outlines and abraded grain surfaces which are the products of eolian abrasion. Lastly, quartz grains from quartz-cemented sedimentary rocks have moderately angular outlines and well-defined quartz overgrowths which are the products of quartz cementation in deep-burial settings. However, the results also show that this critical assumption is false for quartz sand grains that are released from water-transported sedimentary rocks with no secondary quartz. The shapes of quartz sand grains from such sediment sources are relict--that is, they are inherited from the sources of the water-transported sedimentary rocks; they do not reflect the genesis or lithology of their present souces; and they are commonly indistinguishable from quartz grains from other sources.