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Abstract A survey of the optical and E.P.R. absorption in a large number of diamonds revealed that most natural diamonds contain substitutional nitrogen exclusively in non-paramagnetic, platelet form. These diamonds are classified as Type I a. A small group of transparent natural diamonds contain dispersed, paramagnetic nitrogen. The optical properties of these diamonds are unlike those of other diamond types and were hitherto unknown. It is proposed that these diamonds be classified as Type I b. The absorption continuum in the ultra-violet and visible region and the infra-red bands in the region of single single-phonon absorption, characteristic of Type I b diamonds, are shown to be due to paramagnetic nitrogen in solid solution. Synthetic diamonds are found to be generally of Type I b, whereas natural Type I b diamonds are rare exceptions. This is attributed to the growth history of the specimens.
Dyer et al. (Thu,) studied this question.