Magnetism in rocks can provide information primarily about the minerals present in the rock, particularly those that contain magnetic properties, such as magnetite or hematite. By studying the magnetic properties, geologists can infer various characteristics about the rock's formation, its thermal history, and the tectonic processes that may have affected it.
Additionally, magnetism can also provide insights into the age of rocks, especially in the context of paleomagnetism, which studies the magnetic orientations preserved in rocks to understand the historical changes in the Earth's magnetic field and plate tectonics.
So, in summary, magnetism in rocks reveals information about both minerals and potentially age.