The present invention provides a system, method, and apparatus for producing a dimensionally stable extruded board product for use in surface coverings. The board product may be a basalt casting powder-based product for use as a substrate for wall or other surface coverings or may be used as a core layer in a modular floor covering unit. The modular floor covering unit comprises multiple layers. The layers are a thin cut stone veneer layer, a core layer with a density similar to that of the thin cut stone veneer layer, and an optional magnetically receptive underlayment layer. Other layers or combinations of layers may also be used.