Proustite (Ag3AsS3), a new acentric crystal for optical mixing, has been synthesized as large transparent single crystals. It is uniaxial, having point group symmetry 3m. Proustite transmits from 0.6 to 13 &mgr;m, and has a large negative birefringence (n0−nE∼ 0.2). Using second‐harmonic generation from a 1.152‐&mgr;m laser, we estimate the nonlinear coefficients to be|d22P|=50|d36KDP|and|d31P|=30|d36KDP|. New optical mixing possibilities arise from the exceptionally wide range of processes that can be phase‐matched and the large nonlinear coefficients.