What is the relationship between the speed of crystallization of the magma and the hardness of rocks?

hardness of rock is in direct relation to the rate of crystallization. cooling has inverslely proportional to the crystal growth which decied the hardness of the rocks. rocks like volcanic rock cools rapidly are hader than the slowly cool plotonic rocks.

Hardness has related directly to the crystals/grains size of rocks, and the rate of magma cooling decides the size of these grains. If magma cools slowly then the crystals have enough time to grow and become large, and if magma cools quickly the crystals have no time to form - so they are small. Thus, dense fine-grained igneous rock is stronger than coarse-grained igneous rock.