Architecture of small urinary stones were studied with special attention to the crystal-matrix interrelations. Scanning electron microscopic observation revealed that, regardless of their crystalline components, all the stones had one or multiple core areas which were composed of randomly aggregated crystals. The core area was easily identified because it had free space among the crystals. The core area was enveloped by the laminated mantle layers which were composed of compactly packed

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... , although small free space was found occasionally between the layers. In such space, randomly oriented crystals were observed as in the core area. Transmission electron microscopy (TEM) showed that the organic matrix of the mantle layers was laminated itself and filled the intercrystalline space. From these findings the following conclusions were drawn. There seems to be two mechanisms of stone growth. The first is the crystal adhesion. In this case crystals are randomly oriented, and there is free space among the crystals. Consequently, the stone construction is brittle. The second is the crystal growth in the gel-state matrix which continuously encrust on the stone surface. This type of growth mechanism resembles that of the pearl. It allows no free space among the crystals resulting in a firm architecture. Such a crystal-matrix interrelation should be the mechanism of concretion.