If you want to do some terrain for a barren waste table, or are trying to simulate the surface of Mars, then volcanic rock is a great material to use. It is most commonly found in a rust color but can also be found in a black or even sometimes in a gray color. Large bags of landscaping gravel can be purchased at most gardening centers for a very reasonable price. They are also used in many gas barbeque grills if you want some a little larger. The rocks can also be painted if you can’t find a suitable color, however since they are very porous it can be a little difficult.
The most commonly available size is this landscaping gravel.
Because these rocks are so soft, it is simplicity itself to make your own ground cover using them. Some of the rocks only require you to apply some weight to them to get them to crush. Others will require several taps with a small hammer. Either way the fragments will explode out in all directions, so we recommend doing it inside of a container with some sufficiently high walls. We also recommend doing it on a concrete surface since most other surfaces tend to get dented by the use of a hammer. In a very short time you can easily accumulate as much ground covering material as you need. We like to keep the different sized particles all mixed in together, but by using a screen it is also possible to sort by size.