Abstract The concept of proppant diagenesis has been discussed for several years. Detailed studies and analysis have been conducted to better understand hydrothermal degradation of alumina-based proppant, which has sometimes led to a significant loss of fracture conductivity. Also, testing has been performed in laboratories at high temperatures to accelerate the rate of reactions over short durations. Many of these tests have been reported in the literature and have provided significant insight to various geochemical reactions that occur within the fracture and the possibility of the proppant pack being unstable under laboratory conditions. However, it is equally important to look for evidence of proppant pack instability under reservoir conditions. Attempts were made to evaluate proppants that were flowed back during production from fracture stimulated reservoirs. A sample of a manufactured proppant obtained for evaluation was taken from a well that had a sandstone-dolomite lithology, while natural sand (quartz) proppant samples were obtained from different fracture stimulated wells where shale was the dominant lithology (Barnett shale). Various evaluation techniques were used, such as looking at pH of the fluids, inductively coupled plasma (ICP), X-ray diffraction (XRD), X-ray fluorescence (XRF), and loss on ignition/total organic content (LOI/TOC). Scanning electron microscope coupled with energy dispersive X-ray (SEM-EDX) proved to be beneficial for detailed analysis and helped improve understanding of the structures (growths) on the proppant surface. Extensive efforts were made to finalize best-method testing procedures along with different analytical methods. Efforts were also made to validate some of the results generated in the laboratory under near-reservoir conditions of temperature and test duration. This paper presents data from the analysis for flowback manufactured proppants and fracturing sands that confirm the phenomena of proppant diagenesis under reservoir conditions, as well as laboratory conditions that replicate pseudo-reservoir conditions as closely as was practical.