Emilio • Link
I've had a couple of these myself. They form naturally as certain substances reach saturation point in the bladder, at which point some of the excess crystallizes to form stones. They can be jagged or smooth, small enough to pass naturally or much too large. The body has various ways to prevent crystallizing from happening, the best of which of course is to keep a nice high ratio of fluids to solids in the urine. Drinking lots of water and not going to excess with substances, such as alcohol, that tend to dehydrate you are thus the best ways for those susceptible to prevent stones.
What contributes to stones depends on what the stone is made of. The most common ones (and my own particular ones) are made of calcium oxalate, which can form relating to eating lots of dairy products (rich in calcium) and lots of meat or green leafy vegetables (increasing oxalate levels), while vitamin A or the B-complex vitamins can help prevent them. The sheet of foods to watch out for that my doctor gave me contained most foods except onions, though, so the most important things for prevention seem to be drinking lots of fluids and keeping a balanced diet of
many types of food.
This said, there are also various non-dietary conditions that can contribute to stones as well. For me, though, the dietary stuff has been most important (I knew I'd been drinking a lot of alcohol and relatively little water before my last stone, so I wasn't too surprised when it arrived). The NIH has quite a detailed site on stones here: