Luminous quasars are known to display a sharp steepening of the continuum
near 1100A. This spectral feature is not well fitted by current accretion disk
models, unless comptonization of the disk emission is invoked. Absorption by
carbon crystalline dust has been proposed to account for this feature. Ton 34
(z=1.928) exhibits the steepest far-UV decline (F_nu prop nu^{-5.3}) among the
183 quasar HST-FOS spectra analyzed by Telfer et al. It is an ideal object to
test the crystalline dust hypothesis as well as alternative interpretations of
the UV break. We reconstruct the UV spectral energy distribution of Ton 34 by
combining HST, IUE and Palomar spectra. The far-UV continuum shows a very deep
continuum trough, which is bounded by a steep far-UV rise. We fit the trough
assuming nanodiamond dust grains. Extinction by carbon crystalline dust
reproduces the deep absorption trough of Ton 34 reasonably well, but not the
observed steep rise in the extreme UV. We also study the possibility of an
intrinsic continuum rollover. The dust might be part of a high velocity outflow
(13000 km/s), which is observed in absorption in the lines of CIV, OVI, NV and
Ly_alpha.Comment: 7 figures, to appear in A&
