Rock things

So i am staring to clean out some ash for the first time and i have been finding these things, this is the bigest of them, 3"x4"x2", but there's alot of them, i have been buring some old growth mapple, is this some kind of mineral from the wood or am i screwing something up?

Clinkers. From the impurities in your wood. Coal fired furnaces and forges(especially) get them.

Sweet! just another nail in the coffin for how bad a** this stove is.

Huh. Never had a clinker in my wood stove. Plenty when I was burning corn, though.

I have seen these in my stoves. More so in my last catalytic stove than my current stove though.

I found a small one once!