Which of the following seed is used in production of engineered diamonds ?

Question

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Correct — A, Graphite. A lab-grown diamond is made of exactly the same thing as a mined one, pure carbon crystallised in the cubic form, so the process must start from a carbon material — and of the four options only graphite is carbon at all. In the high-pressure high-temperature route, which remains the cheapest and most widely used, a press generates a pressure of roughly 5 to 5.5 gigapascals at a temperature above 1,400 degrees Celsius. Inside, a solvent-catalyst metal — nickel, cobalt or iron — melts, dissolves the high-purity carbon source, which is graphite, and carries the dissolved carbon to small diamond seeds, where it precipitates as diamond. The other production route, chemical vapour deposition, works at low pressure from a gas: methane and hydrogen in a ratio of about one to ninety-nine are ionised into reactive radicals, and carbon deposits on a substrate while the hydrogen etches away any non-diamond carbon that forms. One point of vocabulary deserves stating honestly, because a careful student will notice it. Strictly, the seed in both processes is a small diamond crystal, and graphite is the feedstock rather than the seed; the Commission has used 'seed' loosely to mean the starting material. But the option set forces the intended reading: the other three are diamond simulants, materials that look like diamond and are not carbon, and none of them takes any part in growing a diamond.

Why the other options are wrong — (b) A diamond simulant — zirconium dioxide in a cubic form, stabilised with other oxides and grown as an inexpensive stand-in for diamond in jewellery. It contains no carbon, so it can neither become a diamond nor supply one with material. The reason it is on the page is that it is the imitation stone most people have actually heard of.  (c) Colourless corundum, aluminium oxide, used as another cheap look-alike stone. Like cubic zirconia it is a simulant, sold in place of diamond, and it plays no role in diamond synthesis. Simulant and synthetic are the two words this question is really separating: a simulant imitates diamond, a synthetic diamond is diamond.  (d) Silicon carbide, and the most convincing of the three simulants because its brilliance is close to diamond's — but it is a silicon compound, not a carbon crystal, and it is a substitute for the finished stone rather than an input to making one.

Concept: Carbon has several allotropes — diamond, graphite, fullerenes, graphene, nanotubes — that differ only in how the atoms are bonded. In diamond each carbon is tetrahedrally bonded to four others in a rigid three-dimensional lattice, which makes it the hardest natural material and a good conductor of heat but not of electricity. In graphite the atoms form flat hexagonal sheets with weak forces between the layers, which is why it is soft, slippery and electrically conducting. Turning one into the other is a matter of forcing carbon into the diamond arrangement, and the two industrial routes do it in opposite ways: HPHT reproduces the conditions of the deep mantle, while CVD builds diamond atom by atom from a gas at low pressure. What must not be confused with either is the class of diamond simulants — cubic zirconia, moissanite, white sapphire, glass — which are different substances chosen for a superficial resemblance. Lab-grown diamonds are graded like natural stones and are distinguished from them only by the growth traces instruments can detect, not by composition.

State PCS 2025 Q121 — Science & Technology PYQ with Answer

State PCS 2025 Q121 — Science & Technology PYQ with Answer