Is basalt harder than granite?
Both basalt and granite are types of rock, and a rock is composed of multiple minerals – except for some rocks composed of essentially one mineral like limestone (calcite) or quartzite (quartz). Minerals are crystalline compounds with known characteristics such as hardness, specific gravity, etc. Therefore the hardness of the rock depends on the exact location on the rock you are testing the hardness, as it will vary from mineral to mineral. Some minerals even have differential hardness. Granite is composed of quartz, feldspar, and usually some ferromagnesian minerals such as biotite or hornblende (depending on the definition of a granite). Basalt is composed mostly of plagioclase (a feldspar) and pyroxene. Each of those minerals has a different hardness, and geologists never use the hardness of a “rock” to determine what kind of rock it is. Basalt is usually much finer grained than granite so it would be virtually impossible to determine the hardness of the individual mineral crystals
