HPHT-grown yellow diamond is a laboratory-grown diamond produced by the high-pressure, high-temperature method whose yellow body colour arises from isolated substitutional nitrogen atoms that the carbon lattice has not had geological time to aggregate into pairs or larger clusters. This makes the stones Type Ib, a category that is common in synthetic diamond and rare in nature, where most yellow diamonds are Type Ia with aggregated nitrogen. Yellow HPHT growth is the simplest and least expensive output of the HPHT process because it requires no nitrogen getter; nitrogen is in fact already present in the metal flux and the air around the cell.

Production

HPHT yellow diamonds are grown in iron-cobalt or iron-nickel flux at conditions near 5 to 6 GPa and 1300 to 1500 degrees Celsius, generally for several days per cycle. A diamond seed sits at the cool end of a temperature gradient and a graphite source at the hot end; carbon precipitates from the saturated flux onto the seed. The crystals show cubo-octahedral morphology with extended cube faces, sometimes including small {113} faces.

Russian, Chinese and Indian producers have manufactured yellow HPHT melee in industrial quantities since the 2000s, and the material now dominates the small-size laboratory-grown yellow market. Larger sizes are also routinely produced; faceted stones above five carats appear regularly at the major laboratories.

Identification

Identification is straightforward for a working laboratory:

• Cubo-octahedral growth-sector zoning, frequently visible to the naked eye in larger stones as cross-shaped or geometric colour distribution and clearly imaged under DiamondView or comparable deep-ultraviolet instruments.
• Metallic flux inclusions, dark under reflected light, magnetic in many cases, and unique to HPHT growth.
• Strong yellow or yellow-orange fluorescence to long-wave and short-wave ultraviolet that follows growth-sector boundaries; phosphorescence is sometimes present.
• FTIR spectra showing characteristic Type Ib absorption, particularly the C-centre band at around 1130 cm and the related 1344 cm feature, often without the aggregated A and B centres typical of natural yellow diamond.
• Photoluminescence spectra at low temperature confirming HPHT origin, including diagnostic nickel-related and silicon-related centres.

The combination of Type Ib character with cubo-octahedral zoning is, on its own, near-conclusive for HPHT origin. Major laboratories report the stones as "laboratory-grown diamond" and apply a girdle laser inscription identifying them as such.

Trade considerations

HPHT-grown yellow diamonds offer fancy yellow colour at a fraction of the cost of equivalent natural fancy yellows. Disclosure is mandatory under FTC, CIBJO and trade-organisation rules. The market segment is distinct from natural fancy yellows, which retain a substantial premium driven by rarity and provenance, and the two should never be priced or compared as if they were interchangeable. For buyers using fancy yellow melee in pavé applications, HPHT-grown material has become the dominant practical option below the half-carat size simply because consistent natural fancy yellow melee is increasingly hard to source.