Mixed rare earth iron garnet crystals often show a growth anisotropy, an orthorhombic contribution to their magneto‐crystalline anisotropy determined by the growth facet from under which the material is taken. We have observed the optical birefringence associated with this anomalous anisotropy. The anisotropy and the birefringence are both found to fall off monotonically with increasing distance from the growth surface. Two other contributions to the observed birefringence are attributable to localized strain and the so called linear magnetic birefringence.