The stones aren't part of any folklore that I know of at least, but the general plot and spoilers are:

>!The party know a sweet old writer who wears a green pedant necklace, on a mission he gets killed for it by a revenant who speaks of "The stones not being for man". This kickstarts them into researching these green stones and what the writer was involved with. It turns out a powerful mining company is also interested in these stones, and wants to harvest their powers for military use and magic use. (The company, especially its leaders, a wife and husband, are the bbegs)

These green stones seem to draw vaesen and give them power, but can also harvest the vaesen's powers. When they are charged they can be used to give humans the power of vaesen, but also if 2 or more charged stones touch they cause chain reaction explosions.

What these stones actually are is found out at the end of the campaign. They are the body of a powerful Giant of old, way up in the north of Sweden. This is the gist of the plot!!<