In okasankaar I'm able to place this stone, which I don't remember ever receiving, at an altar near the village. But nothing seems to happen?
If I interact with it again, I take it away from the altar and then I can't interact with the altar again. Maybe it's a bug?

https://i.imgur.com/AdWDjyg.png

Well, I'm extremely late to this one, and assume the OP either figured it out or moved on. But in case anyone else has this question:

The stone is actually the light for the lighthouse. There is a quest to replace it with a new light. If you can find a new light, then you can place that one in the lighthouse to finish the quest. Talking to people around Okasankaar should uncover this quest line.