Bladder stones (also called uroliths or cystic calculi) are rock-like formations of minerals that develop in the urinary bladder. There may be a large, single stone, or a collection of stones that range in size from sand-like grains to gravel. Many times, there is a mixture of both large and small stones present.
https://vcahospitals.com/know-your-pet/bladder-stones-in-cats

