The most extensive, commercially useful source of thorium as monazite sand occurs in India at Travancore coast. Thorium is a chemical element with symbol Th and atomic number 90. It is a radioactive actinide metal. Thorium is one of only three radioactive elements that still occur naturally in large quantities as a primordial element. Monazite is a brown crystalline mineral consisting of a phosphate of cerium, lanthanum, other rare earth elements, and thorium.