That reminds me of a "clinker". It was what was left from burning dirty coal in a furnace. They are like stone and won't burn, you have to remove them.
Welcome to the site eca. I would think you should put this one in the rock forum. There a few in their that probably can tell you what it is. Most here are describing Slag from what I have read. Slag is a bi product from a old ancient steel factory used back in the Civil War times to make steel with for guns and such. I find slag in all colors here where I live at in Ga. Good Luck, rock