25 Sunstone pendants from India with silver pin and eye.

The mineral oligoclase or sunstone is a sodium calcium aluminum tectosilicate with 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 cleavage according to crystal plane [001] and good according to [010]. The average density is 2.65 and the hardness is 7. The crystal system is triclinic and the mineral is neither radioactive 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 location of oligoclase is Tweedestrand in Norway. The Vikings probably used oligoclase as a navigational instrument to determine the position of the sun at sea in cloudy weather. This mineral can visualize polarized light that forms circles around the sun due to refraction in the atmosphere. The crystal doubles the rings. If both are equally visible, the crystal is pointed straight at the sun. The rings are also visible after the sun has set for a while.