  We investigate the dust extinction properties in the host galaxy of the
Gamma-Ray Burst (GRB) GRB 050904 at z=6.29 by analyzing simultaneous broad band
observations of the optical and UV afterglow at three different epochs. We show
that the peculiar afterglow spectral energy distribution (SED) observed at 0.5
days and at 1 day after the burst (1.6 and 3 hours rest frame) cannot be
explained with dust reddening with any of the extinction curves observed at low
redshift. Yet, the extinction curve recently inferred for the most distant BAL
QSO at z=6.2 nicely reproduces the SED of GRB 050904 at both epochs. Our result
provides an additional, independent indication that the properties of dust
evolve beyond z~6. We discuss the implications of this finding within the
context of the dust production mechanisms through the cosmic ages.
