HPHT-grown blue diamond is a laboratory-grown diamond whose blue body colour is produced by the deliberate inclusion of boron in the growth cell during high-pressure, high-temperature synthesis. The colour mechanism is identical to that of natural Type IIb diamond: isolated substitutional boron atoms accept electrons from the carbon lattice and absorb light at the long-wavelength end of the visible spectrum, leaving a blue residual transmission that ranges from grey-blue at low boron concentrations through to a more saturated blue at higher levels.

Production

HPHT growth uses a metal-solvent system, typically iron-cobalt or iron-nickel based, in which a small diamond seed crystal sits at one end of a temperature gradient and a graphite source at the other. Pressure and temperature conditions are held within the diamond stability field, generally around 5 to 6 GPa and 1300 to 1500 degrees Celsius. Carbon dissolves in the metal flux at the hot end and precipitates onto the seed at the cooler end. To produce a blue stone a boron-bearing additive is loaded into the cell; the cell is also configured with a getter to capture nitrogen, because nitrogen donors compensate boron acceptors and can suppress the blue colour or push it toward grey.

Most commercial HPHT blue diamonds are grown by the cubo-octahedral seeded method developed in the 1990s and refined by producers including Sumitomo, AOTC and a number of Chinese and Russian growers. Rough crystals show characteristic cubo-octahedral morphology with extended cube faces, in contrast to the dominant octahedral habit of natural diamond.

Identification

Identification of HPHT-grown blue diamond is well established. Diagnostic features include:

• Anomalous colour zoning following the cube-octahedral growth sectors, often visible as cross-shaped or hourglass patterns under magnification or in DiamondView fluorescence imaging.
• Metallic flux inclusions of iron-cobalt or iron-nickel alloy, which are magnetic and which the natural diamond lattice does not host.
• Phosphorescence after short-wave ultraviolet excitation, often greenish or bluish, lasting tens of seconds to minutes; long-lived phosphorescence is shared with natural Type IIb stones but the spectral signature differs.
• Fourier-transform infrared spectra showing Type IIb character with boron-related absorption and the absence of the natural diamond's nitrogen aggregation features.
• Photoluminescence features at low temperature that distinguish HPHT growth from chemical-vapour-deposition growth, including specific Ni-related and Si-related centres.

Major laboratories including GIA, IGI and HRD report HPHT-grown blue diamonds on their standard synthetic-diamond reports. Every faceted HPHT blue diamond submitted to GIA receives a laser inscription on the girdle indicating laboratory origin.

Market position

HPHT-grown blue diamonds occupy the upper tier of the laboratory-grown blue market because the growth conditions are demanding and yields are lower than for colourless or yellow output. Wholesale prices are nonetheless a small fraction of natural Type IIb prices: a one-carat fine blue HPHT-grown diamond trades in a different market segment from a comparable natural stone. Disclosure is mandatory under FTC, CIBJO and major-trade-organisation guidelines, and the trade term is "laboratory-grown diamond" with the colour-cause noted where relevant.

For the working jeweller the practical points are that HPHT-grown blue diamonds are real diamonds with the same hardness, refractive index and dispersion as natural diamond, that they must be disclosed at every point of sale, and that an unirradiated, unannealed HPHT-grown blue diamond should always carry a current report from a recognised laboratory.