Calculi formed in the kidneys (stones, gravel or sand according to size) are usually composed of calcium oxalate, the smallest of which are commonly termed bladder sand. Note the concentric growth layers. The photomicrograph was taken through crossed polarizers with a full-wave retardation plate inserted into the light path between the specimen and the analyzer. The objective was a planapochromat 10x/0.32 NA coupled to an achromatic condenser and mounted on a Zeiss Ultraphot III microscope equipped with an automatic 35-millimeter photohead. The film was Fujichrome Velvia. (25x)