At Ruby Hill near Bingara they were found in a breccia filling a volcanic pipe.
It was discovered by chance in 1858, having been until then hermetically sealed by a mass of limestone breccia.
Sandstones, quartzites, conglomerates and breccia make up the formation.
Cohen, who regarded the pipes as of the nature of a mud volcano, and the blue ground as a kimberlite breccia altered by hydrothermal action, thought that the diamond and accompanying minerals had been brought up from deep-seated crystalline schists.
Red granite was obtained from the First Cataract, breccia and diorite were quarried from very early times in the Wadi Hammamat, on the road from Coptos to the Red Sea, and porphyry was brought, chiefly in Roman times but also in the prehistoric age, from the same region at Jebel Dokhn.