In the stone industry, we often use the Mohs hardness scale to help our customers better understand the strength of their slab, comparably to other slabs. For instance, your granite is harder than your quartzite, or your quartzite is harder than your marble. The only problem with this method is that the Mohs Hardness Scale was developed with the strength of a single mass element. So the difference is that a solid rock of quartz is pure quartz, but a slab of quartzite can contain several different minerals so understanding the strength of the slab is a little more complicated.
The Mohs Scale is used to determine the relative hardness of a mineral as a comparison to another mineral. The scale was created in 1812 by the German geologist and mineralogist Friedrich Mohs. The method of comparing hardness was created by seeing which minerals can visibly scratch others.
So for example, when you are dealing with things like granites it is hard to give an exact hardness, because granite is a rock composed of multiple minerals. Granite is typically composed of quartz (Mohs-7), potassium feldspar (Mohs-6), and biotite and muscovite micas (Mohs-2.0-2.5). The varying amounts of these minerals would made it difficult to assign a specific Mohs hardness to granite generally. The hardness of granite is greater than many other rock types because of its interlocking mineral structure which is a result of the growth of mineral crystals out of a molten state. This means that it become harder than other materials because of the temperature and pressure it was compressed at.
In conclusion, each slab can have different hardness properties. A quartzite can be just as hard as a granite or as soft as a marble due to the mineral and chemical make up of the slab. This goes for marbles and granites. All of these terms are relative to the chemical makeup. When choosing a stone slab, there is no guarantee of hardness so if you are looking for a guarantee your best bet is finding the and researching the best sealer.