This is the Motrak Models HO Scale Marblehead Gravel Loads for Athearn/MDC 52' Gondola (2-Pack).
These granite loads are made out of Hydrocal base and real granite glued on top. Hydrocal plaster will break if dropped. If it breaks, you can simply use white glue to join the pieces back together. If the gravel load is chipped, you can spray diluted white blue with water and sprinkle some fine grain gravel. Some loads might need some trimming for easy drop-in applications.
Before I ordered these, I was reluctant--after all, it is just gravel. But the price seemed good. The info said it was made from real granite, I think, glued to the top. When I got them, I was amazed. You can tell that they are real granite (or stone) and they really pop out in your hopper or gondola. Instead of a plastic imitation of something, it gives the appearance that your car is actually filled with real, large sized gravel.