The ferroelastic domain texture of the orthorhombic phase of single crysstals of YBa₂Cu₃O_7−x grown in gold crucibles and containing 1.2 at.% Au has been studied by reflected polarized light microscopy. The structure is charaterized by stacks of lamellar domains with a high density of walls, as also found in crystals grown in other crucibles, but with particular properties, possibly related to the high amount of gold incorporated. The unusual optical anisotropy and the related contrast formation of lamellar stacks of domains with a high density of walls (“puzzle domains”) are qualitatively interpreted by extending the model of so-called form-birefringence (the case of transmitted light) to form-bireflectance by the introduction of complex refractive indices.