Sunstone from India, smooth stone

The mineral oligoclase or solar stone is a sodium-calcium-aluminum-tectosilicaat having the chemical formula (Na, Ca) (Si, Al) 4O8. It belongs to the feldspars. The colorless, greenish, gray or brown oligoclase has a glass luster, a white stripe color, a perfect crystal cleavage plane according to [001], and a good according to [010]. The average density is 2.65 and the hardness is 7. The crystal system is triklien and it is neither radioactive mineral, nor magnetic. The name of the mineral oligoclase is derived from the Greek words oligos and klasein "little cleavage." Oligoclase is a common feldspar in metamorphic and igneous rocks as pegmatite. It is part of the plagioclase series (albite-anorthite). The type of location is oligoclase Twede Beach in Norway. Presumably the Vikings oligoclase used as a navigation tool to sea in cloudy weather to determine the position of the sun. This mineral can make polarized light which is visible through the refraction in the atmosphere circles around the sun. The crystal doubles the rings. If both are equally strong visible crystal straight at the sun. The rings are also visible when the sun is already just below.