Bladder stones generally form in the bladder itself. They arise as a result of urinary stasis such as in bladder outflow obstruction (enlarged prostate) or in patients with a neurogenic bladder (loss of bladder function due to spinal cord injury/disease). Those with bladder wall abnormalities (ureterocele, diverticulum) or those with recurrent urinary infections are also at higher risk of forming bladder stones.
When seen on an abdominal/pelvic X-ray they are often multiple and rounded.