English: Interference figures for a biaxial mineral, viewed along one of its two optic axes. The curved shape of the dark arc (the "isogyre") is characteristic of biaxial minerals - though the degree of curvature will change as the microscope stage is rotated, and at some orientations the pattern will resemble the "maltese cross" pattern of a uniaxial mineral. The left hand image illustrates the figure alone; the grey patch at the centre indicates the low first order (grey) birefringence colours seen here (the order of the colours seen would in reality increase away from the center, but these colours are not shown). The two right hand figures show the effect of adding a sensitive tint plate to the setup, replacing the grey at the centre with second order blue and first yellow birefringence colours. The polarity of the yellow and blue reveals whether the mineral being viewed is optically "biaxial positive" (top) or "biaxial negative" (bottom), which can be a key property in identifying the mineral (or investigating its composition).