There is no easy answer here. It could be slag imbedded in a concretion of sand and clay. Slag is waste metal, it would be magnetic. Sand and clay sediments form rock all the time. Differing conditions cause it to become rock over time, like sandstone, or the calcium in solution becomes calcite when the proper conditions are met. Another example is alluvial flow from a glacier picking up sand and other sediments and becoming puddingstone. Inside a puddingstone you will see other rocks around which the puddigstone was formed. As you can see, in the included picture there are stones which were picked up in an alluvial flow including jasper, quartz, a lake superior agate and more.