Most minerals have a hardness of 3 to 7. A penny, hardness of 3, is scratched by a knife blade with a hardness of 5.5. The rapidity of mineral weathering is directly related to hardness- the harder the mineral, the slower it weathers. Specific gravity is simply the relative weight of a mineral compared to an equal volume of water. Cleavage is the way minerals split along planes of weakness.
Cleavage is related to the structure of the mineral and is usually parallel to other mineral faces.
All of these minerals are on display in today's laboratory. Fracture is the breakage of a mineral in a way other than along cleavage planes. Thus, most minerals show fracture rather than cleavage. Several common fracture types are illustrated here. Several minerals exhibiting these types of fracture are on display in today's laboratory.