According to Greek legend, boats sailing the Mediterranean were at risk of punctures from the spikes of the shellfish that provided the famous Tyrian purple dye. Nothing speaks more eloquently of the great toughness of these natural materials. Now chemists in the United States report that they can use the spiny secret of the mollusc shell to control the growth of crystals, offering delicately tailored composite materials.

Mollusc shells – typified by nacre (mother-of-pearl) – are crazy-pavings of hard slabs of mineral, stacked in many layers like a pile of slates. Between each layer is a sheet of soft, organic, protein-based material that helps to bind the slabs together. But the protein sheets may be more than just glue. They are responsible for ensuring that the crystalline mineral grows with the right shape: into flat slabs rather than compact, faceted blocks. This brickwork arrangement, interleaved with softer material, makes the shell very fracture-resistant as well as simply hard.

What is more, the organic material in the matrix of nacre seems to influence the precise arrangement of atoms in the mineral. It is composed of calcium carbonate, the fabric of chalk and limestone. But calcium carbonate comes in two different crystalline forms, with different stacking arrangements of the calcium and carbonate ions: calcite and aragonite. Calcite is the most stable form at sea-water temperatures, but the slabs of nacre are composed of aragonite. Somehow the proteins help to stabilize this form of the mineral instead of calcite.

The protein sheets are studded with acidic groups which, to a calcium ion, can look rather like carbonate ions. If the acidic groups are arranged in a regular lattice within the protein sheets, calcium ions might mistake the surface of the sheet for the surface of a carbonate crystal, and would stick to the sheet to form the first layer of a genuine crystal with a structure determined by the nature of the organic 'template'.

It is still not clear whether this is really what happens in nacre. But the idea has inspired researchers to try to make composite materials using the same principles. Composites of hard and soft materials might offer the same combination of hardness and toughness that we see in mollusc shells – all the more so if the shape, structure and orientation of the crystals can be precisely defined.

Previously, researchers have managed to achieve some control over these factors by using polymers or films of soap-like molecules on the water surface to guide the growth of crystals. But now Haimanot Bekele of the Scripps Research Institute in California and colleagues report in the 1 august issue of the Journal of the American Chemical Society that they have achieved genuine mimicry of nature by designing and synthesizing protein-like molecules that control the orientation of crystals of cadmium sulphide grown from solution.

The organic 'templating' molecules are peptides – short chains fashioned from much the same building blocks as the long folded chains of proteins. The peptides pack together in orderly layers whose pleated structure is very similar to that adopted by proteins in the organic sheets of nacre. The synthetic peptides are encouraged to take up this arrangement by virtue of their 'hairpin' shape; this enables them to line up side by side.

A single layer of the peptide molecules contains regularly spaced acidic groups whose spacing almost matches that of the cadmium ions on one face of cadmium sulphide crystals. The crystals therefore grow with this face flush against the organic film.