A variety of feldspar containing thin platelets of copper or haematite. These fine inclusions reflect light, producing a glow that appears to come from within the stone. The effect—aventurescence—gave it the name "sunstone."

Oregon is the only known source of sunstone with copper inclusions. In highly transparent crystals, the interior appears to burn red when light passes through.

Viking navigators reportedly used sunstone to locate the sun on overcast days. Recent optical research confirmed that feldspar's birefringence can indeed function as a polarising filter to indicate solar direction. Legend, caught up to by science.