Granite is a popular and enduring material used globally for construction and residential surfaces like kitchen countertops. While consumers select this rock for its appearance, its reputation for longevity is the deciding factor for high-traffic areas. Understanding its resistance to wear, particularly scratching, requires looking to the standard geological measure of durability: the Mohs scale of mineral hardness.
Defining the Mohs Scale of Mineral Hardness
The Mohs scale is a qualitative system used to assess the scratch resistance of minerals, ranging from 1 to 10. German mineralogist Friedrich Mohs developed this relative index in 1812 using ten common reference minerals. The fundamental principle is that a harder material will visibly scratch a softer one, but the reverse will not occur. This is an ordinal scale, meaning it ranks materials by scratch resistance, but the jump in absolute hardness between numbers is not uniform.
The softest mineral, talc, is assigned a 1, while diamond, the hardest known natural substance, holds the top rank of 10. For context, an average human fingernail registers at about 2.5 on the scale. Window glass typically falls around 5.5 to 6, while the common mineral quartz is set at 7. Geologists use these fixed points for quick identification and comparison of various natural materials.
The Hardness Range of Granite and Its Components
Granite typically exhibits a Mohs hardness that ranges between 6 and 7. It does not possess a single, fixed number because it is an intrusive igneous rock formed from magma that cooled slowly beneath the Earth’s surface. This slow cooling results in a heterogeneous composition of interlocking mineral crystals. The overall hardness of the rock is therefore an average of its constituent minerals.
The primary minerals in granite are quartz and feldspar, along with smaller amounts of mica and amphibole. Quartz is the defining mineral for scratch resistance, registering a 7 on the Mohs scale. Feldspar, often the most abundant mineral, is slightly softer, usually falling between 6 and 6.5. Because the hardest component, quartz, is distributed throughout the stone, it effectively dictates the surface’s overall scratch resistance.
Real-World Durability: What Granite’s Mohs Number Means for Daily Use
The Mohs 6 to 7 rating translates to an exceptional level of everyday durability for granite surfaces. This hardness means the stone is resistant to abrasion from most common household items. For instance, stainless steel knives typically register around 5.5 to 6.5 Mohs. A granite counter is generally hard enough to resist scratching from a dropped or carelessly dragged knife blade.
The most significant threat to the surface is not metal, but common silica-based dust and sand. Quartz is abundant in sand and dust particles, and since it is a 7 on the Mohs scale, these abrasive grains can potentially scratch the surface if rubbed across it. The primary maintenance concern is keeping the surface free of gritty, quartz-rich debris. Sealing granite protects the stone’s microscopic pores from staining liquids, not its surface from physical scratching.