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Understanding Hardness: Measuring Minerals and Natural Stones

When we measure the hardness of a mineral or natural stone, what we’re really measuring is its relative resistance to scratching. This is typically done by scratching one material against another of known hardness. In this post, we discuss the hardest natural stone and how common countertop materials stack up.

The Hardest Natural Stone Based on the Mohs Hardness Scale

Named for German mineralogist Friedrich Mohs, the Mohs Hardness Scale is used to assign a numeric value between 1–10 to minerals and objects based on their ability to scratch softer substances or their resistance to being scratched. Out in the field, geologists can use common objects with known values on the Mohs scale, such as glass, a copper penny, or a fingernail to test minerals by attempting to scratch them.

The higher the number on the Mohs scale, the harder the substance. Diamonds, the hardest known material on Earth, come in at a 10 on the scale. A human fingernail is a 2.5, while talc—the softest mineral—sits at the bottom with a Mohs hardness of 1.

How Hard is Quartzite?

Quartzite, which contains at least 90% quartz, comes in at an 8 on the Mohs scale. This gives it a slight edge on granite, making it the hardest natural stone countertop commonly available. Quartzite is so hard, in fact, it’s not advised to use knives directly on the surface—not for fear of scratching the countertop, but because true quartzite is more likely to dull a knife blade! One should, however, refrain from cutting directly on any countertop, no matter how hard the material.

Quartzite and granite have similar properties, and both materials should be sealed regularly to protect against staining. Unlike materials containing calcite, granite and quartzite are resistant to acidic substances and will not etch with exposure.

How Hard is Granite?

Granite is by far one of the most popular surfacing materials on the market. It’s also one of the hardest natural stones—surpassed only by quartzite and gemstones. Coming in at between 6–8 on the Mohs hardness scale, granite countertops are favored for their durability, hardness, scratch-resistance, ease of maintenance, and aesthetics. As a reference point, a steel knife sits at a 7 on the scale.

How Hard is Marble?

Marble is an elegant, luxurious stone that’s been lauded for its beauty for centuries. It is, however, one of the softer materials. Composed of metamorphosed limestone and containing mostly recrystallized calcite or dolomite, marble sits between 3–5 on the Mohs scale. As calcite is usually one of its main components, marble is also vulnerable to etching when it comes into contact with acidic materials like juice, wine, sauces, and certain cleaners.

Marble is a porous stone and needs to be sealed regularly to protect against staining and moisture damage. While marble requires diligent maintenance and care against scratching or making contact with hard objects, it can elevate the aesthetics of any space where lifestyle and daily traffic allow for it.

How Hard is Engineered Quartz?

Quartz countertops are often confused with quartzite, but there’s an important distinction to be made: quartzite is a natural stone surface, while quartz countertops typically refer to engineered quartz, which means they are man-made. Quartz countertops are made of roughly 90% quartz that is ground up and bonded with resins, polymers, and pigments.

Engineered quartz comes in at around 7 on the Mohs scale of hardness. This surface material is non-porous and doesn’t typically require sealing. Although engineered quartz is comparable to granite in hardness, the resins that bind everything together can take on damage, such as melting from exposure to extreme heat.

How Hard are Porcelain Slabs?

Several factors can affect the hardness of a porcelain slab, but unglazed porcelain typically sits at a 7 on the Mohs hardness scale. This makes porcelain an excellent option for those seeking a middle ground between durability and price. Because its hardness can vary, it’s important to work with a reputable manufacturer when shopping around for porcelain tiles or large-format porcelain slabs.

How Hard is Onyx?

Onyx falls between 6 and 7 on the Mohs hardness scale, putting it on or slightly under the hardness level of engineered quartz, porcelain, and granite. However, it is a more brittle natural stone that can be prone to scratching and cracking when used as a surfacing material or in high-traffic areas. Onyx slabs tend to come with a fiberglass mesh backing to help strengthen and hold them together.

How Hard is Soapstone?

Soapstone has high talc content and only has a rating of 1 on the Mohs hardness scale. Although soapstone is dense and nonporous, giving it excellent water-repelling properties, it can be vulnerable to scratches and chips. Soapstone intended for practical use in kitchens and bathrooms will feature less talc and be harder. Working with a reputable dealer will ensure that soapstone is a good match for your lifestyle.

In conclusion, understanding the hardness of minerals and natural stones is essential when choosing materials for countertops or other surfaces. The Mohs Hardness Scale provides a reliable framework for comparing and evaluating the hardness of various substances.

Quartzite stands out as the hardest natural stone countertop commonly available, with a rating of 8 on the Mohs scale. Its exceptional durability and resistance to scratching make it a popular choice. Granite follows closely behind, ranking between 6 and 8, offering a balance of hardness, aesthetics, and ease of maintenance.

Marble, although prized for its elegance, is a softer stone, ranging from 3 to 5 on the Mohs scale. It requires diligent care to protect it from scratches and etching caused by acidic substances. Engineered quartz, with a hardness of around 7, offers a non-porous surface and low maintenance requirements.

Porcelain slabs, onyx, and soapstone provide additional options with varying degrees of hardness. Porcelain slabs typically rate around 7, striking a balance between durability and price. Onyx falls within the range of 6 to 7 but requires extra care due to its brittleness. Soapstone, with a hardness of 1, is unique with its water-repelling properties but can be susceptible to scratches and chips.

When selecting a natural stone or engineered surface, it is crucial to consider not only hardness but also factors like porosity, resistance to staining, and specific maintenance requirements. Consulting with reputable manufacturers or dealers will help ensure the best match for your needs and lifestyle.

Ultimately, by understanding the hardness of different natural stones and engineered surfaces, you can make an informed decision and choose a material that suits your preferences, durability requirements, and aesthetic desires.


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