They may be single or multiple, I have seen dogs with as many as 50 individual stones present in the bladder. Some stones have a smooth, polished-feeling surface, while others can be very rough and spiny. Bladder stones are classified as to their mineral content. There are three main types of stones commonly encountered in dogs. Struvite stones are the most commonly found.
Struvite stones occur in conjunction with bacterial infection of the bladder.
The action of the bacteria causes urine to become alkaline, which allows the formation of the stones. Calcium oxalate stones are also found in dogs. Formation of these stones is thought to occur with abnormally high urine concentrations of calcium and oxalic acid. I see these stones most frequently in the schnauzers. Urate stones are the third type commonly found in dogs.