  Classical DQ stars are white dwarfs whose atmospheres contain detectable
traces of carbon brought up to the surface by a convective dredge-up process.
Intriguingly, unlike other white dwarf spectral classes, DQ stars virtually
never show signs of external pollution by elements heavier than carbon. In this
Letter, we solve this long-standing problem by showing that the absence of
detectable external pollution in DQ stars is naturally explained by the impact
of metal accretion on the atmospheric structure of the white dwarf. A DQ star
that accretes heavy elements sees its atmospheric density decrease, which leads
to a sharp drop in the molecular carbon abundance and a strong suppression of
the C$_2$ Swan bands. We show that a typical DQ star that accretes heavy
elements from planetary material generally transforms directly into a DZ star.
