This is the agatey, flinty material that we often find through in Ayrshire. It cracks like obsidian does and can be used to knap arrowheads or tools, although these pieces are small. Occasionally we find these near Alloway in Ayrshire. These two have been tumbled and took a great shine. The podge doesn't shine but does make it flint/chert rather than pure chalcedony.