Rocks come in many varieties, ranging from soft and crumbly to extremely hard and durable. The hardness of a rock depends on its mineral composition and molecular structure. Some rocks contain minerals like quartz, feldspar, mica and calcite which lend them strength. Others may be composed of softer minerals like clay, gypsum or talc. Geologists use the Mohs scale of mineral hardness to quantify the relative hardness of different rocks and minerals. The Mohs scale ranks minerals on a scale from 1 (softest) to 10 (hardest). Diamond is the hardest mineral with a Mohs hardness of 10. The following are 5 of the hardest rock types found on Earth according to the Mohs scale.
Quartzite
Quartzite is a metamorphic rock formed when quartz-rich sandstone is exposed to high pressures and temperatures deep below the earth’s surface. The high heat and pressure cause the quartz grains to fuse together forming an extremely hard and durable rock. Pure quartzite is composed almost entirely of quartz which has a Mohs hardness of 7. This makes quartzite one of the hardest rocks in existence. Quartzite often contains other minerals like feldspar, mica and iron oxides which can lower its overall hardness slightly. But pure quartzite approaches the hardness of quartz at around 7 on the Mohs scale. Quartzite frequently occurs in shades of white, grey or pink depending on mineral impurities. Its extreme toughness made quartzite a popular building stone in antiquity. Quartzite continues to be used in construction, decor and manufacturing today.
Granite
Granite is an igneous rock composed mainly of quartz and feldspar minerals. These minerals are exceptionally hard with quartz having a Mohs hardness of 7 and feldspar having a hardness of 6 – 6.5. This makes granite one of the most durable natural stones in the world. The typical components of granite are 20-60% quartz, 25-65% feldspar and small amounts of mica and amphibole minerals. Variations in mineral composition and cooling history result in different colors and textures of granite. Slow cooling deep underground leads to large visible mineral crystals while fast cooling at shallow depths results in fine-grained granite. Granite comes in many colors like grey, pink, white and black depending on the mix of minerals present. Granite’s combination of hardness, attractiveness and abundance make it coveted as a construction stone. It is used for buildings, monuments, countertops and flooring.
Basalt
Basalt is an extrusive volcanic rock formed from the rapid cooling of lava rich in iron and magnesium exposed at or very near the earth’s surface. Basalt is composed of the minerals plagioclase, pyroxene and olivine. These minerals have Mohs hardnesses of around 6 making basalt one of the hardest rocks. The fine grained, dense structure of basalt gives it a very high compressive strength which contributes to its hardness. Compressive strength refers to a material’s ability to withstand being compressed. The rapid cooling of molten basalt gives it a fine crystallization that contributes to its hardness and strength. In addition to its hardness, basalt’s dark color, durability and abundance make it useful for construction projects including roads, buildings and monuments.
Diabase
Diabase is an igneous rock similar to granite and basalt. It is composed mainly of the minerals plagioclase and pyroxene. Diabase forms when molten magma cools slowly deep within the earth’s crust unlike basalt which forms from cooling on the surface. The slower cooling of diabase allows larger mineral crystals to form giving diabase its distinctive coarse grain. The components of diabase typically include 50% plagioclase feldspar, 25% pyroxene, and small amounts of olivine, magnetite and apatite. The hardness of diabase ranges from 5 – 6 on the Mohs scale due to the hardness of its mineral constituents. Diabase’s combination of fine grained matrix with larger crystals gives the rock both strength and durability. These properties plus its natural dark coloring make diabase a popular rock for building and construction.
Anorthosite
Anorthosite is a rare plutonic igneous rock composed almost entirely of the mineral plagioclase feldspar. Plagioclase has a Mohs hardness of around 6 making anorthosite a very hard rock. Anorthosite forms deep underground where plagioclase crystallizes from a melt rich in calcium and aluminum. Anorthosite is distinguished from other igneous rocks by its lack of mafic minerals like pyroxene and olivine. The mineral makeup of anorthosite is over 90% plagioclase with trace amounts of olivine, pyroxene, garnet and quartz sometimes present. Anorthosite often exhibits a leuco-cratic texture meaning a light color due to its predominantly white plagioclase crystals. Anorthosite’s high strength, brilliance and natural white color lend it to use as an architectural stone. Famous examples include the Beacon Building in Maine and the Tulip Stairs at the Queen’s House in England.
| Rock Type | Hardness on Mohs Scale |
|---|---|
| Quartzite | Around 7 |
| Granite | 6 – 7 |
| Basalt | 5 – 6 |
| Diabase | 5 – 6 |
| Anorthosite | Around 6 |
Conclusion
The hardness of rocks is an important property with implications for construction, manufacturing and geology. Granite, basalt, quartzite, diabase and anorthosite represent some of the hardest and most durable rocks found in nature. Their crystallization under intense heat and pressure causes the formation of exceptionally robust mineral structures. Hard rocks like these have lent their strength as building materials throughout human history. Their hardness arises from key rock-forming minerals like quartz, feldspar, olivine and pyroxene which measure near the top of the Mohs scale. Granite and related rocks will continue serving as fundamental materials for architecture and infrastructure thanks to their unrivaled hardness and endurance.
