I found many of these nuts in a botanical garden in Sweden (outside, not in a greenhouse) but was unable to find the plant it belonged to. The garden has species from all over the world. I don't recognize it as a native plant, but I could be wrong on that.
I've tried to use various identification webpages to find it, but failed. I would love to know the species, so that I can get more info on the plant and its seeds! The Bic lighter is there for scale.
The nuts (or seeds, or whatever they are) are very hard, like a peach stone or other drupe stones.