In crossed-polar mineral optics, the faintly coloured rings and dark curves produced as a result of retardation when convergent polarized light passes through anisotropic (i.e. double refracting) minerals. The black curves or crosses are called ‘isogyres’. There are two kinds of interference figures: uniaxial and biaxial. When centred, the uniaxial interference figure resembles a black cross with coloured concentric circles; the biaxial interference figure is more complex, with two curved isogyres and coloured elliptical bands. The coloured bands represent zones of equal retardation and tests using accessory plates give optical properties which are characteristic of individual minerals.