80% of all bladder stones (uroliths) in cats are either struvite or calcium oxalate. The remaining 20% are formed from a variety of minerals, each type occuring very rarely, and so I will focus on the two main types.
Struvite crystals form when the urine is not acid enough. These crystals come in two forms, sterile and infection-induced. The sterile form is just precipitated minerals from the urine. They are common in cats 1-8 yrs old, in both male and female. Diets low in magnesium, phosphate, and protein can lead to their dissolving. Prevention includes lowering the urine pH, decreasin urine specific gravity, and decreasing dietary magnesium, ammonium, and phosphates. Increased water consumption also helps prevent formation.
The infection-induced struvite crystals tend to form in very young or older cats. A bacterial infection in the bladder releases urease which then leads to the formation of the crystalline nidus (seed-crystal). The treatment of these crystals is much the same as sterile struvite crystals, except that antibiotics should be used while the dissolution food is being fed. The reason for this is the bacteria trapped in the stone are released as the stone is dissolved and can cause UTI or further stone formation. Antibiotics should continue for 2 weeks past when the stone no longer is visible on xrays or ultrasounds. Prevention of infection-induced struvite crystals does not require any change in diet -- only thorough treatment of any UTI.
Calcium oxalate crystals form when urine is too low in pH. Various medical conditions such as hypercalcemia is associated with an increase risk in calcium oxalate stones. These stones can not be dissolved by diet and must be physically removed. Prevention is treatment of any contributing medical condition and increasing fluid consumption.
In the photo, Photo A shows a typical sterile struvite crystal which is nicely round and smooth. Photo B shows 3 infection-induced struvite crystals -- note that they are smooth but geometrically shaped. Photo C shows a calcium oxalate crystal with typical spikes.
Based on "Update on feline Urolithiasis" by Amanda Callens and Joseph W. Bartges, pp. 499-508, in Consultations on Feline Internal Medicine, v7, 2015