taconite, low-grade iron ore, a flintlike rock usually containing less than 30% iron. Resistant to drilling and to the extraction of its contained metal, the rock was long considered worthless. Experiments begun in 1912 by the American scientist Edward W. Davis and continued by him for nearly 40 years produced the pelletizing method for upgrading the ore. The development of the jet piercer (a high-temperature flame thrower) provided penetration speeds of up to 40 ft (12 m) an hour for blasting holes in the rock. In 1956 exploitation of the vast reserves of taconite in the Mesabi range of Minnesota was begun by some of the largest steel companies in the United States.