The book above was mentioned by @Sociolingo in a recent blog post on Sand Divination in Mali. The following passage is from Chapter 7. Disclaimer: The links to external sites are my own additions.

“My introduction to Cedena, or sand divination, took place in Dakar, Senegal, where the local Islamic culture credits the Bamana (also known as “Bambara”) with a potent pagan mysticism. Almost all diviners had some kind of physical deformity — “the price paid for their power.” One diviner seemed quite willing to teach me about the system, suggesting that it “would be just like school.”The first few sessions went smoothly, with the diviner showing me a symbolic code in which each symbol, represented by a set of four vertical dashed lines drawn in the sand, stood for some archetypical concept (travel, desire, health, etc.) with which he assembled narratives about the future. But when I finally asked how he derived the symbols — in particular the meaning of some patterns drawn prior to the symbol writing — they all laughed at me and shook their heads.”That’s the secret!” My offers of increasingly high payments were met with disinterest. Finally, I tried to explain the social significance of cross-cultural mathematics. I happened to have a copy of Linda Garcia’s Fractal Explorer with me, and began by showing a graph of the Cantor set, explaining its recursive construction. The head diviner, with an expression of excitement, suddenly stopped me, snapped the book shut and said “show him what he wants!”

The book can be purchased at Amazon. For those of you who would like to delve further there’s the International Study Group on Ethnomathematics and yes, of course there’s the obligatory TED talk.