Rusting is a chemical process that typically occurs in iron and its alloys when they are exposed to moisture and oxygen over time. In the context of a rock, rusting specifically refers to the oxidation of iron minerals present within the rock.
The parts of a rock that will undergo rusting are primarily those containing iron-bearing minerals, such as magnetite, hematite, or pyrite. When these minerals are exposed to water and oxygen, they can oxidize and form iron oxides, which manifests as rust. This process can weaken the rock structure and change its color to reddish-brown due to the presence of oxidized iron.