The early stages of formation of the crystalline elements in the continuously forming sea urchin teeth were studied using polarized light microscopy, SEM, TEM and calcite overgrowth. Transient amorphous calcium carbonate (ACC) is initially deposited in the needles and subsequently transforms into a single crystal of calcite. ACC exists in the center of both the primary plates and the needles, even though the surfaces are already well crystallized. TEM images and electron diffraction patterns, as well as calcite crystal overgrowth shows that the highly convoluted primary plate-lamellar needle complex grows into a single crystal of calcite. The calcite crystal c axis is approximately parallel to the slightly curved surface of the primary plate. The needles grow along the unusual [102] direction of calcite. This study demonstrates that highly complex shaped single crystals are produced using a transient amorphous precursor phase. The beneficial properties of the single crystals are then combined with those of a polycrystalline matrix to yield a sophisticated composite functional material. This strategy could conceivably be applied to the production of synthetic materials.