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Kidney filters waste from our blood and excrete in terms of urine. sometimes minerals and salts which go into our body get stuck in kidney.they form together and make a stone in the kidney. The size of the stone too varies .
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kidney stones :- kidney stones are hard mass formed in the kidneys , typically  consisting of insoluble calcium compounds .

Kidney stones often have no definite, single cause, although several factors may increase your risk. Kidney stones form when your urine contains more crystal-forming substances — such as calcium, oxalate and uric acid — than the fluid in your urine can dilute.
     i hope it helps.........................^_^
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