Polycrystalline CVD diamond is a synthetic diamond material grown by chemical vapour deposition in conditions that produce many small diamond crystallites simultaneously, forming a dense interlocking aggregate rather than a single crystal. The polycrystalline form is the dominant industrial diamond product and is used in cutting tools, abrasive compounds, optical windows, and thermal-management applications. It is not used in jewellery; the single-crystal CVD form is the gem-grade material.

Growth process

Chemical vapour deposition produces diamond by activating a hydrogen-and-hydrocarbon gas mixture — typically hydrogen with a small percentage of methane — in a low-pressure reactor and depositing carbon onto a heated substrate. Whether the resulting deposit is polycrystalline or single-crystal depends on the choice of substrate and the growth conditions. A non-diamond substrate such as silicon, tungsten carbide, or a diamond-seeded surface, combined with high methane concentrations and certain growth parameters, produces simultaneous nucleation of many small diamond grains that grow together into a polycrystalline aggregate. A pristine single-crystal diamond substrate, combined with carefully controlled growth parameters, produces a homotaxial extension of the substrate crystal as a continuous single-crystal layer.

The polycrystalline material consists of randomly oriented grains separated by grain boundaries that scatter light and degrade optical clarity. Hardness, thermal conductivity, and chemical inertness remain at near-diamond levels, making the material highly valuable for industrial applications, but the material is opaque to translucent at any commercial thickness and shows no intrinsic clarity advantage over natural industrial-grade diamond.

Industrial applications

The principal commercial use of polycrystalline CVD diamond is in cutting and machining tools, where the extreme hardness and wear resistance of diamond are matched against the heat and abrasion of cutting harder workpiece materials. Polycrystalline diamond inserts are bonded to tool bodies and used to machine non-ferrous metals, composites, ceramics, and dense plastics. Diamond grit, produced by crushing polycrystalline material or grown directly as a powder, is the abrasive used in lapidary laps, polishing compounds, and industrial grinding wheels.

Polycrystalline CVD diamond is also used in thermal-management substrates for high-power electronic devices, in optical windows for high-energy laser applications, and in radiation detectors. The thermal conductivity of diamond — among the highest of any solid — and its chemical inertness make it the substrate of choice in selected demanding applications.

Distinction from gem-grade material

The phrase CVD diamond in jewellery contexts almost always refers to single-crystal CVD material grown for gem use. Single-crystal CVD diamond is optically clear, can be faceted to gem standards, and is the material that has driven the rise of laboratory-grown diamond in the consumer jewellery market over the past decade. The two materials share the same growth principle but produce entirely different products. Confusion between the two is common in journalism and in casual trade discussion; gemmological laboratories and serious dealers maintain the distinction carefully.

Identification

Polycrystalline CVD diamond is straightforward to identify: it is opaque to translucent, shows visible grain structure under low magnification, and is not faceted. The species identification — diamond — is confirmed by hardness and by spectroscopic analysis. Single-crystal CVD diamond, when faceted, is identified by laboratory analysis using photoluminescence spectroscopy and other techniques to detect the characteristic growth signatures.

In the trade

Polycrystalline CVD diamond does not appear in the jewellery trade except as the abrasive material in lapidary tools and polishing compounds, where it is sold as diamond grit, diamond paste, or diamond-loaded inserts. The material's commercial market is industrial: tool manufacturers, thermal-management vendors, and a small number of optical-window producers.

Further reading

• GIA — Diamond reference
• GIA Gems & Gemology — laboratory-grown diamond articles
• International Gem Society — synthetic diamonds