Sandstone is classified as a clastic sedimentary rock and tends to be medium-hard or soft. The hardness and density of sandstone can vary considerably depending on its composition and how compacted it becomes over time.
Sandstone forms when layers of sand accumulate and are compressed under pressure. The sand grains are cemented together by minerals like calcite, clays, or silica. These cementing agents fill the pores between the sand grains and bond them together into a solid rock. The most common mineral in sandstone is quartz, which is composed of silica. Quartz grains are very durable which gives sandstone some hardness. However, sandstone also contains weaker minerals like feldspar, mica, and clays which reduce its overall hardness.
The relative abundance of quartz versus weaker minerals impacts the hardness and density of the resulting sandstone. Sandstone with high quartz content is harder and more resistant to weathering compared to sandstone rich in feldspar or clay. The type and amount of cementing agent present also affects sandstone strength. Sandstones cemented with calcite or silica tend to be harder than those with clay cement.
In addition to mineral composition, the compactness and porosity of sandstone influences hardness. As sand accumulates in layers, the weight pushes out air and compacts the grains together. Compacted sandstone is harder than loosely cemented varieties because there is less space between the tightly packed grains. The more porous and permeable the sandstone, the softer and weaker it will be.
Most sandstone used in construction and building applications falls into the medium hardness range. It is not as soft as shale or mudstone, but also not nearly as hard as granite or quartzite. On the Mohs hardness scale, a measure of mineral hardness, sandstone generally falls in the 6 to 7 range. Quartz alone ranks a 7 on Mohs scale, but the other minerals and pore spaces in sandstone reduce its overall rating. However, well-cemented and compacted sandstone can achieve ratings of 8 or 9 approaching the hardness of quartz.
Sandstone has many useful properties as a building material. Its relative softness makes it easy to cut, shape and carve compared to very hard igneous or metamorphic rocks. Yet sandstone is also hard enough to resist weathering and wear from exposure to the elements. Its texture and colors make it appealing for architectural uses too.
While sandstone is considered medium-hard, its actual hardness varies based on composition. Weak, porous sandstone is soft enough that you can scratch it with a fingernail. Very compact, quartz-rich sandstone requires a pick to break it and can only be scratched with a steel point. So sandstone encompasses a range of hard and soft varieties between the extremes of shale and granite. In construction uses, sandstone provides an attractive and useful balance of workability, durability and hardness.
