Small Unburnt Coal Pieces in Ash Pan??

Hello,
Any ideas on Small unburnt coal pieces that are mixed in with the ash? Possibly an adjustment needed on my Keystoker KA-6 Boiler?

they are most likely rocks or shale

Lightly hit it with a hammer, is it shiny inside?

Post a picture, it's easier to know exactly what you're talking about.

I snapped a piece and it is in fact shiny inside. Not sure what that tells you? Coal? I'll get a picture of it and post it here. I Turned the boiler temp down to about 160 from 180 to see how that works? I read in another post about it today.
Thanks much for you time.

Well what you might have is mixture of good coal and not so good coal. I think what happens is the not so good coal burns up quickly and doesn't get hot enough to ignite the real good stuff... i.e its like throwing pine and oak together and the oine burns up quickly and the oak is just getting started. With the coal though possibly by that time it's already going into the ash pan, just a guess really but I've seen it before and generally the unburned pieces are of high quality. Odd but it happens.

I would watch the fire. If the entire firebed is glowing orange and the feed rate is at it's normal pace and black stones are then found in the ash pan, I don't know how it could be coal.