Biomineralization: A pavement of pearl

How do pearls grow? Both pearls and mother of pearl are made of nacre, one of several substances that molluscs build their shells from. It consists of layers of aragonite crystal, sandwiched between fibrous organic layers that provide it with elasticity. But the whole organic matrix is constructed first, so why doesn't it get in the way of crystal growth? The answer is simple -- a scattering of small pores, which both allow material to be supplied for crystal growth and help align the crystals.