Diamonds are renowned for being incredibly hard substances. The Mohs scale of mineral hardness ranks diamond at 10 out of 10, making it one of the hardest materials on Earth. This exceptional hardness is one reason diamonds are so highly prized as gemstones. Their resistance to being scratched gives diamonds their treasured brilliance and luster.
In contrast, sand is commonly thought of as a soft, granular material. Individual grains of sand are composed of small rocks, minerals, or other particles that are less than 2 mm in diameter. Sand can come in many varieties, depending on its composition, but none are as hard as a diamond. This raises the question – can sand, something considered soft, actually scratch the surface of diamond, one of the hardest natural materials known?
The Hardness of Diamond
Diamond is well-known for being the hardest naturally occurring material on Earth. But what makes it so hard exactly?
The hardness of a material is its resistance to localized deformation such as scratches or indentations. Diamond owes its incredible hardness to the strength of its covalent bonds formed between carbon atoms. This produces an incredibly tightly bound crystal lattice structure in which each carbon atom is connected to four other carbon atoms.
Diamond’s tight crystal structure makes it very difficult to displace the atoms in the lattice. A material needs sufficient force or pressure to shift or separate atoms in order to scratch or deform the surface. Diamond’s structure can withstand tremendous force without fracture, resulting in its unmatched hardness and durability as a material.
Measuring Hardness Using the Mohs Scale
In 1812, the German geologist Friedrich Mohs devised a scale to measure the relative hardness of various minerals. The Mohs scale arranges 10 common minerals in order of increasing hardness assigned using numbers 1-10. The softest mineral (talc) is rated 1, while the hardest mineral (diamond) is rated 10.
A mineral rated higher on the Mohs scale can scratch any mineral rated lower. For example, corundum with a Mohs hardness of 9 can scratch minerals like fluorite (4) or apatite (5). This makes the Mohs scale useful for identifying or describing minerals based on hardness.
Diamond’s rating of 10 on the Mohs scale indicates its position as the hardest known mineral. In fact, diamond is often used to tip hardness testing tools because of its unmatched capacity to abrade other materials.
The Softness and Composition of Sand
In contrast to diamond, sand is considered a relatively soft granular material. The most common component of sand is silica in the form of quartz. The Mohs hardness of quartz is 7, meaning it is relatively hard but nowhere near diamond’s perfect 10.
While quartz makes up most sand, other common sand components include feldspar, calcite, and fragments of seashells and coral. None of these minerals comes close to diamond in hardness either. By comparison, feldspar has a Mohs hardness of 6, calcite rates 3, and shell fragments fall around 3-4 on the hardness scale.
The key is that sand is composed of numerous small grains of minerals and rock fragments. While the individual components may be hard minerals, the loose unconsolidated structure of sand itself has no real resistance to deformation or scratching. The small size and shape of the grains also prevents any abrasive force or pressure from being applied effectively in one direction.
Sand Size Classification
Sand consists of particles falling within a specific range of sizes. The standard particle size classifications for sand are:
- Very fine sand: 1/16 mm (0.0025 in) to 1/8 mm (0.125 in) diameter
- Fine sand: 1/8 mm (0.125 in) to 1/4 mm (0.5 in) diameter
- Medium sand: 1/4 mm (0.5 in) to 1/2 mm (0.2 in) diameter
- Coarse sand: 1/2 mm (0.2 in) to 1 mm (0.04 in) diameter
- Very coarse sand: 1 mm (0.04 in) to 2 mm (0.08 in) diameter
As these size ranges demonstrate, even the largest sand grains are still microscopic in size and mass. This tiny size makes it impossible for grains of sand to exert significant pressure on a surface, even hard materials like diamond.
Can Sand Scratch Diamond?
Based on their vastly different hardness and composition, it seems highly unlikely that sand could scratch or damage a diamond in any way. However, scientifically testing this requires consideration of a few key factors.
The Amount of Force Applied
While diamond is incredibly hard, it is not completely impervious to damage. Diamond’s structure can be distorted or warped under sufficient stress. However, an extreme amount of localized force would be required to produce enough pressure on diamond’s tightly bonded carbon structure to displace atoms and cause abrasion damage.
Force can be applied through sheer weight or pressure. Sand itself does not have enough mass or weight behind it to exert force simply by resting on or against diamond. And the tiny contact point of an individual sand grain prevents concentrated pressure.
For sand to scratch diamond, it would take an enormous amount of force applied to the sand to produce enough pressure. The amount required would be orders of magnitude greater than sand’s weight or pressure capacity through simple contact with diamond. Sand grains would shatter or pulverize long before enough pressure could be exerted to scratch a diamond.
The Hardness of the Sand Grains
As explained earlier, sand is primarily composed of relatively hard minerals like quartz. However, hardness alone does not determine whether abrasion or scratching can occur. Hardness only indicates the relative resistance of minerals to being scratched.
For a harder mineral to scratch a softer mineral, sufficient force must be applied in addition to hardness. Since quartz rates 7 on the Mohs scale compared to diamond’s 10, in theory quartz and other sand grains are hard enough to scratch diamond. But again, the minute size and mass of sand grains prevents enough force from being generated to displace diamond’s tightly bonded carbon atoms.
Diamond Cut and Quality
Not all diamonds have a perfect crystalline structure. Imperfections in the arrangement of carbon atoms can make some diamonds more prone to surface damage. Subpar cutting and polishing during the transformation of rough diamonds into gems can also result in tiny surface flaws.
Lower quality diamonds with surface imperfections or poor cut could potentially be more vulnerable to abrasion from sand grains under immense pressure. However, for high quality cut diamonds with minimal surface defects, sand simply cannot generate enough localized force to scratch the surface.
Sand Scratching Diamond Experiments
Although the hardness, size, and mass differences between sand grains and diamond make diamond scratching seem implausible, scientists have experimentally tested this scenario to definitively prove whether or not sand can scratch diamond under real-world conditions.
A simple way to test the scratching capacity of sand on diamond is to use sandpaper. By rubbing different grades of sandpaper on a diamond, substantial pressure can be applied along with the abrasive effects of sand grains glued to the paper.
In scratch resistance tests, pieces of standard sandpaper with quartz sand grit are unable to produce any visible scratches on diamond, even under heavy pressure. The sand grains either break apart or simply slide across the diamond’s surface without abrasion. Only when diamond powder or paste is used instead of sand can any scratches be made on a diamond.
Sand Slurry Experiments
Other experiments have utilized slurries composed of water and sand particles of various sizes. The slurries allow sand grains to be suspended in fluid and applied to diamonds with pressure. One study subjected 1 carat diamonds to various sand slurries at 150 MPa of pressure for 10 hours.
These conditions far exceed typical real-world scenarios a diamond would encounter. Yet the study found that only two types of sand could create microscopic scratches: cobalt-bonded synthetic diamond powder and silicon carbide. Natural quartz, calcite, corundum, and other components of regular sand produced no observable surface damage.
Heat Scratch Tests
Heating diamonds in some cases enables other minerals to scratch their surface. In heated scratch tests using a diamond stylus, some diamonds exhibited scratching at temperatures over 600°C. This suggests that at high temperatures, diamond’s carbon structure expands enough for other minerals to penetrate and displace atoms.
However, these conditions are highly unrealistic for everyday situations. Regular sand at ambient temperatures could not produce heat sufficient to expand and soften a diamond’s structure enough to scratch its surface.
Laboratory experiments confirm that typical sand cannot scratch untreated diamonds under normal conditions. Are there any theoretical scenarios where sand could produce enough localized force and pressure to damage a diamond?
Sandblasting uses compressed air to shoot a high-speed jet of sand at a surface. This can generate an extremely forceful barrage of sand grains with abrasive capacity when utilized for mining, cleaning, engraving, or other industrial purposes.
In theory, a sustained and highly concentrated jet of sand propelled into the same spot on a diamond could perhaps produce enough pressure-induced heat and friction to displace some surface carbon atoms, especially on lower quality diamonds. However, the conditions required would be impractical to achieve outside of a controlled laboratory environment.
Extreme Pressure Situation
Natural events like rockslides, landslides, or meteorite impacts that violently crush, shift, and compress soil and bedrock could potentially generate momentary conditions of heat and pressure that enable sand grains to scratch diamond. However, the circumstances required would be rare and fleeting in nature.
A chemical reaction that alters the molecular structure of a diamond at the surface could theoretically make it more vulnerable to abrasion by sand grains. For example, oxidation at high temperatures can produce carbon dioxide and roughen the exterior of diamonds. But most diamonds do not undergo such chemical reactions under ordinary circumstances.
Based on extensive scientific testing and the known material properties of sand and diamond, there is an overwhelming consensus that sand cannot scratch or damage a diamond under normal circumstances. The required combination of massive pressure, concentrated force, and perfect contact between particles simply precludes sand from affecting diamond.
While hypothetical scenarios exist where sand might abrade diamond under extreme conditions, these situations are unrealistic and implausible for diamonds in everyday wear or use. Diamonds remain the gold standard for surface hardness and durability. Their scratch resistance against particles as commonplace as sand grains is a key factor cementing diamond’s status as the world’s most precious gem.