Which of the following seed is used in production of engineered diamonds ?
Graphite
Graphite is the starting material — option 3.
Why graphite. Diamond and graphite are allotropes of carbon — the same element in two different structures. A laboratory-grown diamond is real diamond, chemically and physically identical to the mined stone, so it must be made from carbon, and graphite is the cheap and abundant form of carbon available.
| Diamond | Graphite | |
|---|---|---|
| Bonding | Each carbon bonded to four others, sp3, in a rigid three-dimensional lattice | Each carbon bonded to three others, sp2, in flat sheets held together weakly |
| Hardness | Hardest natural substance, 10 on the Mohs scale | Soft and slippery — the sheets slide |
| Electrical conduction | Insulator — no free electrons | Conductor — one delocalised electron per atom |
| Density | 3·5 g/cm3 | 2·2 g/cm3 |
The two methods. In the high pressure high temperature route, graphite is subjected to about 5 GPa and 1,500 °C with a metal catalyst, which converts it to the denser diamond structure — pressure favours the denser form. In chemical vapour deposition, a small diamond seed crystal is placed in a chamber of methane and hydrogen, and carbon atoms released from the gas settle on it layer by layer.
Why the other three are not the answer. They are all diamond simulants — materials that look like diamond but are chemically something else :
| Simulant | Composition |
|---|---|
| Moissanite | Silicon carbide, SiC |
| White sapphire | Aluminium oxide, Al2O3 |
| Cubic zirconia | Zirconium dioxide, ZrO2 |
None contains carbon, so none can be turned into diamond. The distinction between a simulant, which imitates, and a synthetic, which is the real thing made in a laboratory, is exactly what the question is testing.
An Indian note : Surat is a world centre for cutting and polishing both mined and laboratory-grown diamonds, and India removed customs duty on diamond seeds to support the industry.
Hence, the answer is Graphite.
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