Diagenesis happens before metamorphism. It happens at temperatures less than 200 °C and at pressures lower than 3 Kbar, this is a low-pressure, low-temperature alteration process. The process involves chemical and physical changes by compaction, cementation etc. Since there are several different minerals within a rock, the effect will be different on the different kinds of rock. One example is that diagenesis of feldspar will turn in to a clay mineral. In general the diagenetic changes of rock or sediments occur between the time of deposition and until it has solidified.