Taconite is an iron-bearing, high-silica, flint-like rock. It is a Precambrian seimentary rock referred to as a banded iron formation due to the typical alternating iron-rich layers and shale or chert layers. The very finely dispersed iron content, present as magnetite, is generally 25 to 30%. In the late 19th and early 20th centuries, available iron ore was of such high quality that taconite was considered an uneconomic waste product. After World War II, most of the high grade ore in the United States had been mined out, and so taconite was turned to as a new source of iron. To process taconite, the ore is ground into a fine powder, the iron is separated from the waste rock by using strong magnets, and then the powdered iron concentrate is combined with bentonite clay and limestone as a flux and rolled into pellets about one centimeter in diameter that are approximately 65% iron. The pellets are heated to very high temperatures to oxidize the magnetite (Fe3O4) to hematite (Fe2O3) for further processing. Edward W. Davis is credited with developing this process.