It takes some practice to identify cleavage planes in minerals. It is not immediately apparent, but is one of the most diagnostic tools for mineral identification. Fracture is how a mineral breaks when not controlled by cleavage. Many minerals have no cleavage, and so break in an irregular pattern. Other minerals will fracture in a conchoidal pattern, similar to when glass fractures.
Different minerals have different densities, and therefore weigh differently.
This fact is known to anybody who has ever tried to pan for gold. Gold , being 'heavier' than the other rocks and minerals, sinks when in water, making it easy to collect. Geologists measure the ratio of the mass of a mineral to a mass of an equal volume of water. This is the minerals specific gravity . Water has a specific gravity of 1.