Vikings navigated the oceans with sundials aboard their Norse ships. But on an overcast day, sundials would have been useless. Many researchers have suggested that the on foggy days, Vikings looked toward the sky through rock crystals called sunstones to give them direction.
No one had tested the theory until recently.
A team sailed the Arctic Ocean aboard the Swedish icebreaker Oden and found that sunstones could indeed light the way in foggy and cloudy conditions.
OTOH: Unlike with Viking sundials, archaeologists have yet to find any remnants of sunstones. You'd think if they were so important to navigation, someone would have been buried with one in one of those way cool ship burials.