When a translucent crystal is viewed under microscope, light travels through the mineral at a speed which corresponds with its refractive index, as this is affected by the crystal 2180 planes. A uniaxial crystal has a negative optical sign when the velocity of its extraordinary ray exceeds that of the ordinary ray and vice versa. Calcite is negative; quartz positive. For biaxial crystals, the three principal directions of vibration are mutually at right angles.