Seared on the outside, raw in the middle.

I'm stumped.
Found this when I was walking along the bank yesterday evening. It had been used as a surround for a campfire and there was its twin lying next to it; the two obviously split by the heat of the fire.
To my amateur eye it's some sort of iron-ore centre with, with... eh?
What's gone on here? I'm stupidly fascinated!

NB; taken from the bank of the River Severn at Llanidloes. It's a freestone river at this point (grayling zone) with active meanders but established floodplain.