The fracture energy G of nacre (a stacking of aragonite/organic layers) is
much larger than the corresponding energy Go for pure aragonite (although t
he organic cement thickness do is very small: nanometers). We present here
a qualitative explanation for this difference. The basic idea is that a wea
k cement reduces drastically the stress concentration near the fracture tip
. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevi
er SAS.