Coloured scanning electron micrograph (SEM) of a piece of fractured kidney stone from a ureter. Kidney stones are primarily formed by crystallisation of the mineral salt calcium oxalate from urine. They are irregular shaped stones called calculi (calculus) and are composed of random oriented columnar monoclinic crystals of calcium oxalate monohydrate (seen here) with peripheral deposits of fine octahedral crystals of calcium oxalate dihydrate. Patches of microcyrstalline hydroxyl apatite fill internodal regions. Protein matrices can also be associated with the microcrystalline arrays. The hard stones can cause severe pain as they pass down the ureter (urinary tract). Kidney stones may need to be removed surgically using ultrasound. Magnification: x400 when shortest axis printed at 25 millimetres.