References:
Ancient-Origins.net: http://www.ancient-origins.net/ancient-technology/could-ancient-peruvians-soften-stone-00847
Tompkins, Peter, and Christopher Bird, Secrets of the Soil, New York: Harper and Row, 1989; Research on stone softening by the Incas by Dr. Albert Schatz, pp. 121-125;
Wikipedia: No page on stone softening