Nummulites were one of the most abundant and widespread larger benthic foraminifera of the Paleogene, however, confusion remains within the literature as to whether their original test mineralogy was high or low magnesium calcite. As the number of studies using proxies based on Nummulites and related nummulitid geochemistry increase, it is essential to have a firm understanding of test composition to assess preservation within potential samples, and to interpret results. Here we employ a combination of X-ray diffraction, Fourier transform infra-red spectroscopy, and laser ablation ICPMS to determine magnesium content across exceptionally preserved and poorly preserved fossil material as well as modern examples of nummulitids—showing conclusively a primary intermediate to high magnesium calcite composition. This composition appears to be closely related to fluctuating ocean chemistry through the Paleogene. Using these results as an indicator of preservation we examine variation in trace element data across a suite of samples, and introduce the concept of the preservagram, a method of quickly visualizing different styles of carbonate diagenesis. Understanding the original mineralogy of nummulitids and, therefore, the extent to which specimens have been diagenetically altered, is essential as larger foraminifera are increasingly used in geochemical studies.