A standard hardness scale, called Mohs' Scale of Hardness, is named after the scientist Friedrich Mohs. This is simply a scale from 1 to 10, where 1 is the softest and 10 is hardest. Minerals are naturally occurring substances which have a definite chemical composition, and a definite crystal structure. Quartz is one of the most common mineral in the Earth's crust. Other examples of common minerals are mica, feldspar, and calcite.
Limestone is a rock made mostly of the mineral calcite.
The grains of calcite are normally very small and can't be seen with the naked eye. If the limestone is heated or put under pressure, the calcite grains can melt together and form larger calcite crystals. Marble is a metamorphic rock. Most rocks are made up of crystals. Granite is a good example.