"Empty rock" is an ordinary stone. Both ore and stone will sink into the water. But if you break them and put them in a container with foam, they won't sink. The bubble is so powerful. They will hold up small ore particles and small stones and let them float on them. Then the screening work begins.
Small stones will not persist on the foam surface for a long time and then sink to the bottom, while ore particles will not sink. Finally, there is a layer of mineral particles floating in the container, which is easy to pick out.