Kidney stones are solids made of various chemicals which are produced in urine. Kidney stones are usually as hard as hard as rocks but some are made of softer substances.
The stones are initially produced in the kidneys, but may travel further downstream into the ureter (tube between the kidney and bladder), the bladder or the urethra (water pipe draining the bladder externally). Some stones are stuck to the internal walls of the kidney and do not move around, while others are floating loose within the kidney and can change location. Some stones will pass downstream from the kidney into the ureter when they are tiny and may not even be seen or felt. Other stones will only travel out of the kidney into the ureter when they are bigger and may get stuck in the ureter and cause pain.
Kidney stones vary in size greatly from tiny crystals of less than a millimetre (not visible to the naked eye) up to 10 centimetres (cm). They can vary in colour but are usually brown, black or yellow. Some stones are perfectly round, others are triangular, elongated, irregular or sharp.