A polariscope is composed of two polarized filters with their virational plane perpendicular to each other. When the vibrational plane of the polarizers is at right angles, light cannot pass through. This property is used to test whether a gemstone is homogenic or pleochroic. When turning the stone sample in a polariscope, a bright field indicates it is an aphanitic aggregate; black indicates that the sample is homogenous. If the sample blinks 4 times (light and dark) during a 360° rotation, then the sample is non homogenous.