If a mineral breaks into cubes when struck with a rock hammer whats being measured

When struck, the mineral is tested for lattice type. Depending on how it shatters, scientists can determine how the mineral forms on the atomic level by noting the molecular weaknesses of the system. These weaknesses are the points at which the mineral breaks along.

Depending on the type of hammer, they could also be testing the tensile strength and, even possibly, the hardness, depending on the angle of the blow. How hard the rock can be hit without breaking is the TS, and whether or not the mineral is scratched by the hammer is the hardness.

They are all decomposition reactions.