Effective refractive index tensor for weak-field gravity

Gravitational lensing in a weak but otherwise arbitrary gravitational field can be described in terms of a 3 x 3 tensor, the 'effective refractive index'. If the sources generating the gravitational field all have small internal fluxes, stresses and pressures, then this tensor is automatically isotropic and the 'effective refractive index' is simply a scalar that can be determined in terms of a classic result involving the Newtonian gravitational potential. In contrast, if anisotropic stresses are ever important then the gravitational field acts similarly to an anisotropic crystal. We derive simple formulae for the refractive index tensor, and indicate some situations in which this will be important.