Within the scope of the investigation of more than 1,500 urinary calculi by scanning electron microscopy (SEM), some peculiarities arose concerning secondary phase transitions and morphology of some of the crystalline phases present. There is evidence for a phase transition from Ca-oxalate dihydrate to the monohydrate form in oxalate rich stones. This is characterized by a transitional zone, in which often relatively large monocrystals of newly formed whewellite are found, sticking within weddellite crystals with typical dissolution signs. We conclude that the phase transformation is regulated by dissolution and recrystallization. In mixed uric acid / uric acid dihydrate stones, occasionally parallel growth of apparently two different kinds of crystals upon each other can be observed, which possibly are uric acid crystals grown epitaxially on uric acid dihydrate. A few nearly pure whewellite stones with little Ca-phosphate at the central core show tubules which obviously had been part of a calcified renal papilla and so had been acting as a nidus for stone formation. Bacterial footprints as well as calcified bacteria have only been found in apatite containing stones of patients with urinary tract infections by Escherichia coli. These bacteria are likely responsible for the origin of these footprints.