[Kristoffer O]

Nah, not rusty minerals. More like a crying root in the shape of a small human, a silver acorn, a golden apple, a crystal flower, a rainbow liquid, a floating darkness, a pouch with a trapped sound.

But also gems with trapped midsummer sunlight, the size of an egg.

Obviously magical, but only rarely something a king can use to pay his soldiers with. Soldiers are down to earth and sensible and knows that gold is useful. Barking clams are not.