Diamond is one of the allotropes of pure carbon. It is a crystalline covalent solid found in alluvial gravels, kimberlite pipes and glacial tills. Natural diamonds are under intense temperature and pressure conditions that cause the crystallisation of carbon atoms. Synthetically, it is formed from graphite, mostly using the high pressure-high temperature technique.

Overview

The physical properties of a diamond can be attributed to its structure. It is based on a face-centred cubic lattice structure but with extra carbon atoms inside the cube. The carbon atoms are sp3 hybridised and bonded covalently to four other carbon atoms in a tetrahedral structure. The coordination number of each carbon is 4. Thus, diamond is a giant three-dimensional molecule in which each carbon–carbon bond length is 154 pm, forming strong covalent bonds with each bond angle measuring 109.5°. Diamond has an isometric structure, which implies that the carbon atoms are bonded in the same way in all directions.

General Physical Properties of Diamond

The general physical properties of diamond are:

• Pure diamonds are transparent and colourless. It is the impurities that impart colours to them.
• It is of a brittle nature.
•  It has a refractive index of 2.5, which is a very high value. Therefore, it sparkles. 
• It has the highest non-metallic lustre, known as adamantine, due to its ability to reflect most of the light that strikes its surface. 
• It also has relatively high dispersion. The higher the dispersion value, the more colourful flashes the gem can display.  
• It has an average density of 3.5 g/cm3.
• It has a low coefficient of friction.

On the basis of structure, the following characteristics of diamond can be explained:

1. Hardness: As carbon-carbon bonds are very strong covalent bonds, it is expected that diamonds should be hard and should possess high density. It is the hardest naturally occurring substance. Black diamonds are the hardest of all. 
2. Melting point: Its melting point is as high as 3600°C because its melting requires the breaking of strong covalent bonds. 

3. Thermal conductivity: At room temperature, diamond has the highest thermal conductivity, which can be attributed to the presence of strong covalent bonding in its structure and low phonon scattering.

4. Strength: Diamond’s strength can be related to its FCC crystal structure. Each carbon atom in diamond forms a tetrahedral bond arrangement as it is linked to four other atoms by strong covalent bonds. These tetrahedral joins form a three-dimensional structure with no weak bonds, giving rise to the high strength of diamond. It takes a lot more energy to break this material.

5. Thermal expansion coefficient: It depends on the bond strength between the atoms comprising the structure. As diamond consists of strong covalent bonds, its bond strength is high. Thus, diamonds have an extraordinarily low coefficient of thermal expansion, which implies that they can be heated and exposed to extremely high temperatures without expanding that much. It retains its form and strength at high temperatures.

6. Bad conductor: Since all the valence electrons are involved in bonding, leaving no free electrons to conduct the electric current, diamond is a bad conductor of electricity, or in other words, it has high electrical resistivity.

7. Optical transparency/clarity: The internal structure of diamond consists of carbon atoms covalently bonded in a three-dimensional lattice, which causes deep refraction of incident light into the crystal, giving the crystal its characteristic clarity. It has a wide range of optical transparency, ranging from ultraviolet to infra-red.

Additional Physical Properties of Diamonds as Gemstone

The value of a diamond as a gem is determined by its weight and the presence of impurities in it. The light refractive property, which gives diamond its sparkle, makes it popular as a gem. Its quality is determined by the 4Cs: colour, cut, clarity and carat weight. Its weight is given in terms of carats [1 carat = 0.2 g]. The colourless, transparent ones are the costliest because they have negligible impurities and sparkle the most as they are perfectly polished. Its value decreases with the increase in the impurities present in it, as these inclusions restrict the passage of light. When present in large amounts, the impurities significantly demean a cut gem’s appearance.

It is the presence of small traces of metallic oxide and salt that imparts distinct colours to diamonds. These coloured diamonds are called gems. The diamonds of the darker colours, i.e., red, pink, and blue, are extremely rare and therefore very valuable. They can also be grey, yellow, brown, green and orange, and even black. The black diamonds have copper oxide present in them as impurities. They are not used as gems, but they have important industrial uses.   

Industrial Diamonds

Those diamonds that do not meet the gem-quality standards are used for industrial purposes and are referred to as industrial diamonds. Diamond’s hardness and its unique combination of high thermal conductivity with electrical insulation make it a desirable material for major industrial applications. They are used for cutting and drilling rocks, glass, or other diamonds, for making heat sinks, as tip heads in deep boring drills, as low-friction micro bearing in watches, as needles for long-playing record players, and also for making radiation-proof windows for space satellites, to prevent the entry of harmful radiation. 

Conclusion

Due to its structure, diamond is the hardest natural substance. It is an excellent thermal conductor as well as an electric insulator because of its high melting point. It sparkles because of its high refractive index and dispersion power. It has a low coefficient of friction, extraordinarily low thermal expansion coefficient, and a wide range of optical transparency.

Diamond is most popular as a gemstone but is also used to cut other diamonds, making heat sinks, tip heads in deep boring drills, and for making radiation-proof windows for space satellites.