There is no flint to make celts, but quartz and rocks having a slaty cleavage are abundant.
"The Last Journals of David Livingstone, in Central Africa, from 1865 to His Death, Volume II (of 2), 1869-1873" by David Livingstone
Puff-paste is, then, simply an exaggerated case of slaty cleavage.
"Fragments of science, V. 1-2" by John Tyndall
Cleavage, or slaty structure, 176.
"Principles of Geology" by Charles Lyell
Sharpe, on slaty cleavage, 5, 432.
"The Glaciers of the Alps" by John Tyndall
Herschel, Sir J., on slaty cleavage, 472.
"A Manual of Elementary Geology" by Charles Lyell