Kent V. Flannery references the presence of "High concentrations of magnetite chunks and mirrors fragments" at Oaxaca, Mexico.
Kent V. Flannery, “Sampling by Intensive Surface Collection,” in The Early Mesoamerican Village, ed. Kent V. Flannery (Studies in Archaeology; San Diego: Academic Press, Inc., 1976), 59
Our reasons for conducting the intensive surface pickup were to define the approximate limits of the site for each time period, to locate functionally different activity areas where possible, to distinguish between areas of public buildings and areas of ordinary private houses, and to select areas for future excavation. In the process of analysis of the circles, therefore, we had to make the following simplifying assumptions:
. . . .
9. High concentrations of magnetite chunks and mirrors fragments; onyx chips, drill plugs, and polishers, and numerous shell fragments with chert drills or burins in evidence were taken to indicate areas of craft activity.