Diamonds are found occasionally in the United States of America. Diamonds from the Prairie Creek lamproite in Arkansas, USA occur within a north to south corridor of Neoproterozoic-to-mid-Cretaceous magmatism that extends across North America. These diamond-bearing lamproites are unusual because they intrude adjacent to sutured and strongly thinned lithosphere rather than stable within-plate settings and the diamonds themselves provide physical evidence of processes related to diamond formation at the cratonic margin. Indeed, A review of previously published geophysical data, isotopic compositions, inclusion suites and inclusion geochemistry suggest most diamonds were formed in subducted and eclogitic rocks within a highly localised diamondiferous lithosphere beneath the cratonic margin.

The morphology and spectroscopic character of 155 diamonds from the Prairie Creek lamproite suggest typical diamond formation conditions in an otherwise thinned continental lithosphere. Most diamonds examined during this investigation have spectroscopic features indicating strong nitrogen aggregation, a history of thermal perturbation and plastic deformation. Nitrogen contents range up to 1882 ppm and the diamonds preserve ∼70% aggregated nitrogen in the B aggregation state. Furthermore, inclusion elastic barometry and time-averaged mantle residence temperatures suggest most Arkansas diamonds formed at 5.2±0.2 GPa and 1205±63°C (1σ). However, a subpopulation of ∼4% of relatively large and inclusion free, colourless, flattened-to-irregular habit Arkansas diamonds are Type IIa with <5 at.ppm nitrogen. Those stones size, morphology, colour and N content might warrant their inclusion in the class of Cullinan-like, Large, Inclusion-Poor, Pure, Irregular and Resorbed or ‘CLIPPIR’ diamonds. Other diamonds examined commonly exhibit physical evidence of plastic deformation, including brown body colour and deformation lamellae.