Natural, untreated diamonds showing the A, B and C infrared absorptions (“ABC diamonds”), and the H2 absorption

Unusual brown to yellow diamonds of mixed type Ib/IaAB were analyzed. The occurrence of nitrogen in a combination of A, B and C centers directly detectable by IR spectroscopy in natural diamonds is considered to be extremely rare. We propose to call such stones ABC diamonds for short. These diamonds are characterized by a color, luminescence, and anomalous double refraction distribution which results from mixed growth: a center core formed by cuboid growth, rich in nitrogen, covered by an outer rim of “normal” octahedral growth, with much less nitrogen. Nitrogen in the core is present in a more aggregated state than in the rim, which is practically pure type Ib.In the infrared spectra, besides A, B and C 1-phonon absorptions a large number of previously undescribed, tentatively H-related features were identified in some samples; these proposed H-related features were only present in the periphery of the core and the rim of the diamonds, in the core practically no hydrogen was detected. The low temperature Vis/NIR spectra were mainly characterized by two defect-induced centers: the H2 center, and the apparently new 905 nm vibronic absorption with phonon-side bands at 880, 867, 847 nm plus a feature at 806.5 nm. The 905 nm absorption can possibly be attributed to a hydrogen-related center. Although these diamonds present a combination of spectroscopic characteristics (A + B + C center absorptions, H2 absorptions) deemed characteristic of certain HPHT-treated type Ia diamonds, they cannot be confused for such treated diamonds on the basis of their color, color distribution and some spectral peculiarities.