Mg, Sr, and U concentrations are reported for living, Holocene (dead), and Pleistocene Montastrea annularis and Acropora palmata in an effort to characterize starting materials and identify diagenetic effects in unrecrystallized skeletal aragonite. The data indicate that skeletogenesis of minor elements is a taxa-specific process, yielding different Mg, Sr, and U values for each species. During diagenesis Sr concentrations increase and Mg concentrations decrease in both species. U diagenesis appears to be affected by skeletal structure. Establishment of U concentration ranges for modern corals provides a criterion for selecting samples for dating. Concentration changes in Mg and Sr during diagenesis limit their usefulness as paleo-environmental indicators.--Modified journal abstract.