  We find empirically a characteristic sharp peak-flat trough pattern in a
large set of commodity prices. We argue that the sharp peak structure reflects
an endogenous inter-market organization, and that peaks may be seen as local
``singularities'' resulting from imitation and herding. These findings impose a
novel stringent constraint on the construction of models. Intermittent
amplification is not sufficient and nonlinear effects seem necessary to account
for the observations.
