"Sunstone" is a popular current misnomer for other varieties of feldspar, particularly material from India with bright spangles of mica, and for beautiful copper-infused material of many hues from Oregon, USA. Another gem-grade spangled variety occurs in Tanzania.
As used in Viking lore, the word "Sunstone" referred to some kind of stone (believed by some to be the mineral Iolite) that was used to find the sun on cloudy or foggy days. That name has nothing to do with current usage.
Calcite ("Iceland Spar")is an entirely separate mineral. It is also doubly-refractive and splits light into two beams traveling at different angles through it. One of my friends has created this information about it the fabled Viking "Sunstone": The Viking Compass
great link, thanks.