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Luminous quasars are known to display a sharp steepening of the continuumnear 1100A. This spectral feature is not well fitted by current accretion diskmodels, unless comptonization of the disk emission is invoked. Absorption bycarbon 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 the183 quasar HST-FOS spectra analyzed by Telfer et al. It is an ideal object totest the crystalline dust hypothesis as well as alternative interpretations ofthe UV break. We reconstruct the UV spectral energy distribution of Ton 34 bycombining HST, IUE and Palomar spectra. The far-UV continuum shows a very deepcontinuum trough, which is bounded by a steep far-UV rise. We fit the troughassuming nanodiamond dust grains. Extinction by carbon crystalline dustreproduces the deep absorption trough of Ton 34 reasonably well, but not theobserved steep rise in the extreme UV. We also study the possibility of anintrinsic 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 andLy_alpha.

The unusual UV continuum of quasar Ton 34 and the possibility of crystalline dust absorption