Give clues about the cooling speeds of magmas, the presence of water beneath the surface or even the intensity of past contact metamorphism

The grain size

Explanation:

The grain size can provide lot of clues for us when it comes to the formation of the rocks, the conditions in which they formed, and even what happened after their initial formation. The reason for this is that the grains vary in size and shape depending on the conditions in which the rocks formed. If the magma/lava cooled of quickly, the grains will be very small, almost unnoticeable, and that is a sign that the cooling appeared on the surface, very close to it, or in the presence of water. On the other hand, if the magma cooled slowly, the grains will be large and well defined, and that is a sign that the magma was cooling off slowly, deep into the crust, where there's higher temperature and pressure.