Isaac (sometimes Izak) B. Celliers, geologist, was the son of Johannes Francois Cilliers, editor and publisher of De Volksstem and his wife Magdalena Bisseux, and a brother of the Afrikaans poet Jan F.E. Cilliers. In December 1905 he was awarded the diploma in mining engineering by the School of Mines in Freiberg, Sachsen, Germany. He continued his studies at the Albert-Ludwigs-Universitaet in Freiburg im Breisgau, Germany, where he was awarded the degree Doctor of Philosophy (Phil D) in geology in 1907, with a thesis entitled Geologische Untersuchungen in der Umgebung von Eptingen, Basel-Land. His thesis dealt with the geology of an area near Basel, in northern Switzerland, adjacent to the area investigated geologically at about the same time by his friend and cousin, Louis C. de Villiers*.
After returning to South Africa Celliers became a member of the Geological Society of South Africa in 1908 and was still a member in 1914. He was appointed lecturer in geology at the Transvaal University College, Pretoria (later the University of Pretoria), in 1909. However, a few months later he resigned to join Hans Merensky* in a very successful investigation of the diamond mining potential of the Namaqualand coast.