The patent. Edward Goodrich Acheson was granted US 492,767, "Production of Artificial Crystalline Carbonaceous Materials", on 28 February 1893. Acheson had worked for Edison and was trying to make artificial diamonds by heating clay and coke. He got hard iridescent crystals instead, assumed they were a compound of carbon and corundum, and coined carborundum from those two words. The material is in fact silicon carbide, SiC — no aluminium in it at all. The name survived because the Carborundum Company made it a trademark, and a chemically wrong word became the standard trade name for a material now made by the megatonne.

The chemistry, correctly. SiO₂ + 3 C → SiC + 2 CO, at roughly 1700-2500 °C. The carbon monoxide is why ventilation is not optional, and why blue flames dance over an operating furnace. The Acheson process is a resistance furnace: current passes through a granular carbon core buried in the charge, and the core is the heating element. Because heat radiates outward from that core, the ingot forms in concentric zones — graphite nearest the core where SiC has decomposed again, the best crystals in a band further out, unreacted charge at the edge. Industrial practice recycles the outer material into the next run.

Why half a point on the Mohs scale mattered. Silicon carbide is about 9.5 where corundum is 9 and diamond is 10, but Mohs is a ranking, not a scale — the absolute gaps are enormous. Practically, SiC cuts materials corundum merely polishes, it can be made in quantity from sand and coke rather than mined, and its grain fractures to keep exposing fresh edges. Grinding wheels, cutting discs, wet-and-dry paper and lapping compounds all changed within a decade, which meant everything made by grinding got cheaper — including the precision machine parts that the next generation of industry depended on.

Where it went afterwards. Acheson's furnace also gave him artificial graphite, patented separately, when he noticed SiC decomposing near the core. Silicon carbide later turned out to be a wide-bandgap semiconductor: it holds up at voltages and temperatures that destroy silicon, and SiC power devices are now standard in electric-vehicle inverters and solar converters. A material invented as a substitute for diamond dust, named after a mineral it does not contain, ended up in the power electronics of the twenty-first century.

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Carborundum — Silicon Carbide