They specifically chose the Archean North Atlantic Craton because it hosted the oldest known deposits of the gemstone. The research team was delighted to have such a rare discovery on their hands.

Evidence for a long-lived active margin on the Archean Wyoming craton. Geological Society of America Bulletin, 112, 564-578. Chamberlain, K.R., Frost, C.D., and Frost, B.R. (2003) Early Archean to ...