Answer: A kidney stone is formed from crystals that separate from the urine to form a hard mass within the urinary tract. The natural chemicals (inhibitors) in urine that prevent crystals forming do not seem to work for everyone and 'stones' form. If the crystals are small enough, they can travel through the urinary tract without being noticed.
The most common type of kidney stone contains calcium in combination with either oxalate or phosphate. A less common type of stone is caused by an infection in the urinary tract.
-- Consultant Urological Surgeon, Mr Leye Ajayi