The older gen were very practical and I have seen in old building rubble examples where they used gravel instead of proper builders sand or aggregate for cement or concrete in floors, walls and rendering applications.
Noticing on the back there are areas that look grayish so just wondering the orange colour is just surface staining of a lump of cement.
Additionally, the same sandy conglomerate appears in the spaces between the tiles. It is unlikely that this could have happened naturally as it would presuppose that there were open gaps between the tiles originally and in that case one would hardly expect to see the tiles still so neatly aligned.
Even if building rubble, would be fascinating to try and trace where it came from and why it was there.