Usually struvite crystals. Type of crystals can be determined by urine examination by an expert (vet).

Important: Struvite crystals/stones are dissolvable - other kinds of stones may not be dissolvable. So, try treatment that is designed to dissolve them before you go for surgery. Best treatment is "Hill's S/D Feline" specially formulated food that attacks the struvite problem in multiple ways. You should also give your cat lots of fresh distilled war (in her food and separately). If you prefer a more natural kind of food then you should give the cat it together with some acidifying agent (e.g. cl-methionin) and lots of fresh distilled water as always.

More info on struvite crystals here
A natural blend of herbs that can help with struvite crystals is this.

Treatment
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* lots of distilled water. It is crucial that the water is distilled, as normal water (especially tap water) contains mineral salts that can make the problem worse.
* clean, natural, low in mineral-salts wet food (see here for examples). Make it more wet by adding distilled water.
* cl-methionin (this is simply a aminoacid) added to the food daily

The treatment above may in some cases dissolve the stones (especially in the case of small ones).

If hematuria (blood in the urine) continues after that and the reason is not a urinary infection (blood tests can help you find that out) then you have to remove the stones. There are two options:
* surgical removal (big and fairly dangerous operation)
* new method that slowly emerges but is very hard to find (if at all possible) outside the US and even there in very few climics: LASER LITHOTRIPSY which is a type of non-invasive surgery. Basically a microcamera is inserted into the bladder through the urethra and a laser is used to blast-break the stones through the adjoined optical fiber.

Some info on Laser Lithotripsy:

clinic 1

clinic 2

safety

non invasive