From Scenics site: "Our BEST protypical O-Scale Ballast. Great for 2-Rail O-Scale and S-Scales! Our 32oz. Shaker comes with pour applicator cap! #20 grade natural stone ballast accurately graded for the most discriminating protoscale modeler! Lightly blended hues for subtle contrast. Unlike other ballast, our Scenic Express stone is "sealed" so it will not darken when Matte Medium is applied."