There are lots of types of rock and each type has different properties and uses. For example, some are strong and hard but others are soft and sandy. Some rocks are harder than others. For example ...

Although these measures were not all developed for stones, they have been adapted to allow for measurements of fine-grained rocks.” All kinds of different stones were included in the study ...

Rocks loafing about on the surface of Mars have been harboring secrets about the red planet's mysterious past.