THE study of biomineralization is inspiring new approaches to the controlled fabrication of synthetic materials such as nanoparticles, polymer-mineral composites and templated crystals(1-3). Although this biomimetic approach is gaining momentum, the biological mechanisms involved in biomineralization remain relatively unexplored. One major reason for this is the difficulty of analysing biomineralization processes in their native dynamic state. Here we demonstrate that a highly organized composite material-a ’flat pearl’-can be biofabricated on disks of glass, mica and MoS2 inserted between the mantle and shell of Haliotis rufescens (red abalone). We shove that the construction of this material is spatially and temporally regulated and proceeds through a developmental sequence that closely resembles that at the growth front of the natural shell. Recognition of the implanted inorganic surfaces by mantle cells apparently governs a switch, perhaps genetically controlled, from aragonite to calcite biomineralization. Once a partially oriented calcite-protein primer layer has been deposited, there is a switch back to the nucleation and assembly of columnar stacks of highly ordered aragonitic nacre. Thus the presence of an inorganic surface between the mantle and shell of the organism triggers a change in the nature of the mineral phase deposited.