Diagenesis is now recognized as a potentially major source of error in paleoclimatic reconstructions from fossil and modern coral geochemical records. Key to avoiding spurious results caused by diagenesis is thorough screening of coral material prior to geochemical analysis. In this data brief we present color images from thin sections of fossil and modern Porites corals and demonstrate the effectiveness of thin sections in detecting low levels of diagenesis. The images presented here cover a range of coral preservation levels from pristine aragonite to 100% calcite. We particularly focus on samples containing around 1% diagenetic material, a level known to create artifacts in key climate parameters such as sea surface temperature, and close to the detection limits of other screening methods such as X-ray diffraction (XRD). A qualitative scheme is also presented to rate the degree of diagenesis in a coral, where XRD results are not available or where secondary aragonite is present. Overall, this collection of images is designed as a starting point, in combination with other techniques, to assist in identifying and screening corals for diagenesis.