  To explain the unusual non-saturating linear magnetoresistance observed in
silver chalcogenides, the quantum scenario has been proposed based on the
assumption of gapless linear energy spectrum. Here we show, by first principles
calculations, that $\beta$-Ag$_2$Te with distorted anti-fluorite structure is
in fact a topological insulator with gapless Dirac-type surface states. The
characteristic feature of this new binary topological insulator is the highly
anisotropic Dirac cone, in contrast to known examples, such as Bi$_2$Te$_3$ and
Bi$_2$Se$_3$. The Fermi velocity varies an order of magnitude by rotating the
crystal axis.
