Quartz is one of the most abundant minerals on Earth, forming the foundation of many rocks, sediments, and sands. Its widespread presence and resistance to wear give its physical properties a significant impact on geology and daily life. The mineral’s chemical composition, silicon dioxide, provides durability valued in construction materials and fine jewelry. Understanding its scratch resistance clarifies what materials it can mark.
Understanding the Mohs Hardness Scale
Mineral hardness is defined as a material’s resistance to being permanently scratched or abraded on its surface. This property is measured using the Mohs Hardness Scale, a qualitative system developed in 1812 by German mineralogist Friedrich Mohs. The scale uses ten reference minerals, ranging from the softest, talc (1), to the hardest, diamond (10).
The scale is a relative ranking, not a linear or absolute measurement. It is based on a simple test: a harder material will scratch a softer material. For instance, a mineral with a hardness of 6 can scratch all minerals rated 5 or lower, but it will be scratched by any material rated 7 or higher. The Mohs scale is used for mineral identification and to compare the scratch resistance of different substances.
Quartz’s Position on the Hardness Scale
Quartz is assigned a value of 7 on the Mohs Hardness Scale, placing it high in resistance to scratching. This number is used to define a boundary between minerals generally considered “soft” and those considered “hard.” A Mohs rating of 7 or higher indicates a durable material suitable for applications involving frequent wear.
The hardness of 7 results from quartz’s tightly bonded silicon and oxygen atoms. This structure makes quartz highly resistant to abrasion from most common environmental particles and household materials. This durability explains why quartz is prevalent in sand and retains its structure against weathering and erosion over long geological periods.
Common Materials Quartz Can Scratch
Because of its Mohs hardness of 7, quartz can scratch any material with a lower rating, including a wide range of everyday substances and softer minerals. Ordinary window glass, which measures around 5.5, is a common example. The ability of quartz to scratch glass is a classic test for identifying the mineral in the field.
Quartz can also scratch many common metals used in household and industrial applications. Pure copper has a Mohs hardness of about 3, and aluminum is around 2.5. Even unhardened steel, found in knives or common tools, is typically 5.5 to 6.5, making it susceptible to scratching by quartz.
Among other minerals, quartz can scratch substances like calcite (3), the main component of marble and limestone, and gypsum (2). Feldspar, which coexists with quartz in granite, has a hardness of 6 and can be scratched. The presence of fine quartz dust, or silica, is why softer surfaces like granite countertops or glass windows can accumulate microscopic scratches when improperly cleaned or wiped.
Materials Harder Than Quartz
Only materials with a Mohs hardness of 8, 9, or 10 can scratch quartz. These substances are generally rare minerals or artificially produced high-strength materials. The next reference mineral on the scale is topaz, which has a hardness of 8.
Corundum, rated at 9, includes natural rubies and sapphires. These materials are harder than quartz; a sapphire ring stored next to quartz jewelry could damage the softer material. The hardest naturally occurring material is diamond, which tops the scale at 10 and is about four times harder than corundum.
The hardness of these materials has practical implications in manufacturing. Diamond-tipped tools are routinely used to cut and polish quartz slabs for countertops and engineered stone products. Permanent abrasion or damage to a quartz surface requires contact with these select materials.