Diamond is one of the allotropes of pure carbon. It is a crystalline covalent solid found in alluvial gravels, kimberlite pipes and glacial tills. It occurs in all shapes and sizes and is found in South Africa, Brazil, Russia, Australia, USA and India. The Orapa mines in Botswana is the world’s largest, but Russia and Australia lead in diamond production. In India, diamonds are found at Panna in Madhya Pradesh and Golconda in Karnataka. Its sparkle makes it the most desirable gemstone.

Overview of Diamond: Natural Formation

Natural diamonds are under intense temperature and pressure conditions that cause the crystallisation of carbon atoms. It is present below the Earth’s surface at depths of 150-200 km, where the average temperature is 900-1300°C, and the pressure is about 45-60 kilobars. They are pushed up to the surface by volcanic eruptions. 

Under these intense conditions, molten magma within the Earth’s upper mantle expands at a rapid rate. It erupts to the Earth’s surface with diamond-bearing rocks called kimberlite rocks. They are named after the South African Kimberly Mines, where they were first discovered.

Structure

Diamond is based on a face-centred cubic lattice structure 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 a strong covalent bond. Each bond angle is 109.5°. 

Artificial Preparation 

Graphite is used to make synthetic or artificial diamonds. When subjected to very high pressure and high temperatures (HPHT) of about 3000°C, graphite turns into diamonds.

They are physically and chemically similar to natural ones in composition, crystallisation, and isotropic form. The only difference is that the synthetic solids produced under such conditions are rather small. 

Physical and Chemical Properties

1. In their pure form, diamonds are transparent and colourless. Impurities impart colours to them.
2. Diamonds are hard but also brittle. 
3. Due to their ability to reflect light, they have the highest non-metallic lustre. 
4. Diamond has a refractive index of 2.5, making it sparkle. 
5. It also has relatively high dispersion. The higher the dispersion value, the more colourful flashes the gem can display.
6. It has an average density of 3.5g/cm3.
7. It is insoluble in any solvent. 
8. Though diamond can withstand heat, prolonged heating can change it into graphite. 
9. It burns in air at 900°C to form carbon dioxide.
10. At room temperature, it is one of the best thermal conductors.

On the basis of structure, a diamond’s characteristic properties can be explained as follows: 

• Hardness: As the carbon-carbon bonds are very strong covalent bonds, diamond is hard and possesses high density. It is the hardest naturally occurring substance. The black ones are the hardest of all. 
• Melting point: Its melting point is 3600°C, which is very high. This is because melting diamond requires breaking its strong covalent bonds. 
• Bad conductor: All of diamond’s valence electrons are involved in bonding, leaving no free electrons to conduct electric currents. Hence, it is a bad conductor of electricity.

Value as a Gem

A diamond’s value as a gem relies on its weight and impurities. The 4Cs determine a diamond’s quality: colour, cut, clarity, and carat weight. Its weight is expressed in terms of carats [1 carat = 0.2 g]. 

Colourless, transparent diamonds are the costliest because they have negligible impurities. They are also perfectly polished to be highly reflective and sparkle the most. The value of a diamond decreases with an increase in its impurities. 

Types

It is the presence of small traces of metallic oxide and salt that imparts distinct colours to diamonds. Darker colours, i.e., red, pink, and blue, are extremely rare. DIamonds can also be grey, yellow, brown, green, orange, and even black. 

Black diamonds have copper oxide impurities. They are not used as gems, but they have important industrial uses.

Uses

Pure diamonds are used as gems in jewellery due to their brilliant shine. 

Impure or synthetic diamonds are used:

• For cutting and drilling rocks, glass, or other diamonds. 
• As tip heads in deep boring drills. 
• As bearings in watches.
• As needles for long-playing record players. 
• For making radiation-proof windows for space satellites. 

Conclusion

Diamonds are crystalline covalent solids of pure carbon. In nature, they are formed over millions of years under extreme conditions of pressure and temperature. Synthetically, diamond is formed from graphite using the HPHT method. It has a face-centred cubic lattice structure with sp3 hybridised carbon atoms, each covalently bonded to four surrounding atoms, leaving no free electron. 

Due to its structure, it is the hardest naturally occurring substance, with a high melting point. It is an excellent thermal conductor but a poor electrical insulator. It burns in the air at 900°C to form carbon dioxide. 

Diamond sparkles because of its high refractive index and dispersion power. It is most popular as a gemstone but is also used to cut other diamonds and make radiation-proof windows for space satellites.