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Are diamonds the hardest substance on the earth?

This is common knowledge that the structural composition of diamonds is based on carbon. Carbon is present everywhere around us to inside us, up in the atmosphere and beneath the earth’s surface. Diamond is found in kimberlite, a volcanic rock 150 km deep under the earth.

The extreme temperature and pressure inside the earth bring the carbon atoms closer to each other. This results in a strong coalition of the carbon atoms in the crystal lattice. 

Each carbon atom in the crystalline matrix of a diamond has four electrons in its outermost shell. These electrons are exchanged with four neighboring atoms to form very tight covalent bonds that result in a rigid tetrahedral crystal. The successive compression of the structure continues to harden the stone. The process goes till the well-shaped hardest diamond is formed and extracted.

The carbon matrix absorbs the pressure by distributing it throughout the crystalline matrix that holds its shape and structural integrity. This makes diamond the hardest substance on earth since no other gemstone has been naturally processed at such extreme heat and pressure under the earth’s surface. Furthermore, the fewer the elemental impurities in the diamond, the purer and transparent it is. 

Diamond vs. graphite

Substances like charcoal, black coal, graphite, and diamond are all the polymorphs of carbon. And these differ in arrangement and packing of carbon atoms in their crystal lattice. Among these, diamond and graphite share the same chemistry but differ in their physical and structural properties. Diamond is hard and transparent to a large range of wavelengths; from the ultraviolet into the far infra-red. On the other hand, graphite is soft and opaque. 

The crystallization of diamond is isometric, and that of graphite is hexagonal. Graphite has a layered, hexagonal structure. Strong covalent bonds between carbon atoms are present in each layer, but there are weak forces between layers. In contrast, a diamond has a strong network of bonds in its crystal, called diamond cubic. 

The bonds in graphite are of sp2 order in which the atoms are bound to three nearest neighbors at 120 degrees. In diamond the order is sp3 that forms a rigid tetrahedral structure.  Each atom binds to four nearest neighbors. Thus, of all known substances on the earth, diamond has the greatest number of atoms per unit volume. This is why it is the hardest substance on earth. Naturally, graphite is the most stable form of carbon. However, it converts to diamond under high temperature and pressure.

According to Professor David Phillips, diamond is usually 99.7 percent carbon. The remainder consists of impurities of other elements that impart diamond its colors. Looking at the periodic table, one can guess if a diamond has traces of boron; adjacent to carbon in the periodic table, its color would be blue. And the rare pink color is due to the conundrum. 

Sometimes, the diamond’s crystal lattice experiences molecular deformities while being in the earth’s mantle. This leads to the distortion of light that results in the brown color. Diamonds formed 150 km deep in the earth’s crust is an old myth. Today, synthetic diamonds have been created in laboratories using high temperature and pressure. And they are indistinguishable.

Hardness of diamond

The hardness of any substance depends on the bonding in its crystal lattice. However, the hardness of the gemstones is a special attribute in gemology (the study of gemstones) that offers its definition based on scratch-resistance of the gemstone. Any substance can be scratched by another if it is equal or greater in strength on the hardness scale. 

Since diamond has a hardness of value 10 on the Mohs scale, the highest value, it cannot be scratched by anything except diamond. The Mohs scale is a qualitative scale that characterizes the scratch resistance of various minerals based on the ability of a harder material to scratch a softer material. 

The downside to all-carbon matrix structure is that it can be cut or shattered by striking it along certain angles, the property known as a cleavage plane. Before the technological advancements in the tools for gemstones, identifying cleavage points on a raw diamond took months to years. Diamonds have four cleavage angles and can be chipped and cleaved with the right amount of focused force. It is not the Toughest gemstone like Jade, but it is the Hardest gem on the Mohs scale. This means the diamond is durable and resistant to wear and tear.

If a diamond is the hardest substance on earth, how do jewelers cut diamonds?

If the diamond is the hardest substance on earth, how do jewelers cut it into different shapes? The fact is diamond is brittle along certain cleavage planes, the property called anisotropic. This means diamonds are harder along some directions than others, and this fact is what makes it possible to cut and polish them: some of the tiny crystals will be oriented along those harder directions. 

The diamond is loved for its brilliance and luster; diamond cutters harness these properties by cutting diamonds along weak cleavage planes and enhance the reflectivity of light. For a rough diamond, it can take weeks or months to identify a particular cleave plane.  When identified, the jewelers create a notch with another diamond, as it can be cut by an equally stronger substance, and then cleave it with a steel blade.

Even polished diamonds can break! Missing a piece from either crown or girdle of the engagement ring is a common practice. Diamond can always chip at the right angle. 

Diamond is no longer the world’s hardest substance

Diamonds have lost the privilege to be the hardest substance on the earth. Scientists have discovered two substances; wurtzite boron nitride (w-BN) and lonsdaleite or hexagonal diamonds, a substance found in meteorites, which are harder than diamonds. Only small amounts of both substances exist naturally. Wurtzite boron nitride is formed during volcanic eruptions and Lonsdaleite when meteorites hit the earth. However, scientists have calculated that both have greater indentation strengths than diamond.

A study published in Physics Review Letters by physicists from the Shanghai Jiao Tong University and the University of Nevada, Las Vegas, explains that this case study is the first of its type where any material has surpassed the strength of the diamond under similar conditions. The extreme conditions under indenters transform the wurtzite boron nitride and lonsdaleite into super-hard substances than diamonds. 

Since wurtzite boron nitride and lonsdaleite have only minor differences from the diamond in bonding arrangement, the pressures under indenters induce structural phase transformation of these two into stronger structures. Under compressive pressures, the strength of wurtzite boron nitride reaches 114 GPa (Giga Pascal) compared to 97 GPa (Giga Pascal) of the diamond, 18% stronger than diamond under the same conditions. As for lonsdaleite, it acquires an indentation strength of 152 GPa that is 58% greater than the diamonds. 

 Lonsdaleite is stronger than wurtzite boron nitride because it comprises carbon atoms, and wurtzite boron nitride is made of boron and nitrogen atoms. The carbon-carbon bonds are stronger than boron-nitrogen bonds. This research will provide new approaches for designing super-hard materials. 

Bottom line

When it comes to strength, there are many strong materials in the world. However, the hardness of a diamond as a gemstone is based on its scratch ability. Diamond has a rigid carbon to carbon bonding which makes it scratch-resistant and hardest among all gemstones. 

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