A frustule is the intricately patterned, silica-containing cell wall of a diatom. Its two principal overlapping halves fit rather like a lid and a box, with connecting bands. Pores and ridges differ among diatom groups, making the mineral structure useful in microscopic identification.
How can a biological cell make a patterned mineral structure under mild conditions? Isolated diatom peptides called silaffins caused silica nanospheres to form rapidly when added to a solution of silicic acid. This demonstrated that cellular organic components can directly promote mineral formation.[ref]
Polycationic peptides from diatom biosilica that direct silica nanosphere formation by N. Kröger and others. 1999, abstract: cell-wall peptide isolation and silica precipitation
The experiment accounts for one chemical capacity; a complete frustule also requires control over where that capacity acts and how the resulting material is organized. New to topics? Read the docs here!