Octocorals sclerites are ideal structures to study the emergence of complex shapes from particular arrangements of crystallites in biominerals. Sinularia polydactyla sclerites have been studied by polarizing microscopy, scanning electron microscopy, and electron backscattered diffraction. Small sclerites (<100 µm) are simple mesocrystalline structures, with similarly oriented submicrometer crystallites arranged in elongated fibers, with only a low degree of ordered misorientations between them. Large sclerites (~2 mm) are composite structures: (1) at their center, small proto-sclerites (≤ 20 µm) are present and act as nuclei around which growth proceeds. (2) The large sclerite axial frame is made of crystallites with c axes arranged in three directions, at an inclination of about 22° from the long sclerite axis. The axial frame displays a center of symmetry and crystallites are arranged in opposite trihedral arrangement with respect to this center. (3) Inside the large sclerite, deeply rooted tubercles develop with crystallite c axes close to perpendicular to the long sclerite axis. The tubercles grow by a branching process and display crystallite misorientations ordered around the three a axes of the hexagonal unit cell of calcite. Crystallites in both the frame and the tubercles form trigonal inverse pyramids resulting from sudden or progressive changes of crystallite orientations. A crystallographic model emphasizes the importance of the trigonal inverse pyramid as a structural pattern. Concerning the sclerite morphology, small crystallite sizes, ordered misorientations and mesotwinning are important features to achieve concave shapes. The sclerite surface morphology results from the regulation of mesocrystalline growth and patterning by cells or vacuoles. In this respect, the sclerite morphology is the product of internal and external forces, among which crystallographic order and molding play important roles. 2