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Normal urine is always grossly super-saturated in regard to the stone-forming salts, which are kept in solution by the action of the colloids. This action is best explained by the theory of adsorption.
The amount of the stone-forming salts which can be held in solution depends on the surface area of the colloid, and therefore on its state of subdivision. Precipitation of these salts is due to failure of the colloid to hold them in solution. It may be due to an insufficient quantity, but is more probably due to coagulation of the colloid.
When precipitation occurs in the urinary passages, the crystals are usually retained in the lower calyx of the kidney. A crystalline deposit tends to grow into crystalline concretions under the action of surface energy, thus forming true primary calculi. Stones of this type soon irritate the wall of the cavity in which they are contained and cause a reactionary exudate. The laminated stone is formed by continued deposition of crystals, coupled with rhythmic precipitation of a foreign colloid derived from the exudate. Stones originating in infected media are formed in a similar manner.