It's long been believed that the mineral iolite (cordierite) was used for Viking navigation. A friend who comes from strong Nordic roots and is an accomplished gemologist/mineralogist put together this nice explanation on her website: Viking Sunstone
Thanks.
Thanks. It’s been over 40 years since I took my introductory mineralogy classes; and never kept it up.
I always think of “sunstones” as those artificial glass teardrop things filled with bronze or copper fillings.