Viking «sunstone,» also known as Iceland spar, was a crystal that allowed Vikings to determine the position of the sun even on cloudy or foggy days, aiding them in navigation. This mineral, a type of calcite, is believed to polarize light and create a visible point of light that indicates the direction of the sun.
Iceland calcite, also known as optical calcite, is a transparent, colorless variety of calcite characterized by its high birefringence. This property, birefringence, causes images to double when viewed through it. Historically, it is believed to have been used by Vikings for navigation, helping them determine the position of the sun on cloudy days, and today it is used in precision optical instruments such as microscopes.
