This isn't exactly fantasy, since by all accounts it was a real thing used by the Vikings, but it is my favorite bit of real world fantastic natural occurrences.
First of all, "Sunstones" sounds awesome. It sounds like a treasure, and they truly were in the ancient world. Unless you already know what they did, you're probably guessing their purpose wrong. And that makes them even more fun in a fantasy world. I imagine a player spending hours casting "Detect magic" on these things. Maybe even breaking them open to try and figure out what they do.
If you had a sunstone in front of you, it would probably look like a glossy rock. It would be unlikely for you to stumble on why the Vikings valued them so highly.
They were most likely tourmaline or cordierite crystals but the less impressive ones may have been only calcite. They were virtually magic to a seagoing nation because if you point a properly cut crystal at the sun, even when it's overcast it will glow with the polarized light that passes through it. This is unbelievably useful to sea navigation if you don't have a compass.
I love to see that kind of treasure in a fantasy game. It's subtle in it's effect but when used properly, can have huge implications
I love the idea of the planet on which the players inhabit having a will of its own and sunstones being the manifestation of that will. The planet feels that its North American continent needs to be discovered, so it shudders with a little quake, exposing a new hillside, which has sun stone rock on it. The planet, whose will has a geological timescale, can set things in motion, favouring one species over another.
It's wonderful that the scientists can now say that sun stones aren't myth and that there is a plausible, repeatable, understood scientific explanation.
Well done, Vikings, you continue too rock. Except when you torched the North East*, that was less good.
* Of Britain, because I'm a bit British, you see. Not America. They might have torched the North East of the USA but we don't know for sure. We are sure that Northumbria took a battering.[/edit]
Although Irish and Scottish by decent, A good portion of my genome is Viking so even though they ransacked and pillaged my ancestors on the one hand, they are my ancestors on the other. Family reunions are difficult.