Answer: thoria

AnswerCrossword Clue
THORIAWhite, earthy powder used for optical glass
thoriawhite, heavy, water-insoluble powder, ThO2, used chiefly in incandescent mantles, as the Welsbach gas mantle
thoriaan oxide of thorium
thoriaa powdery white oxide of thorium ThO2 used especially as a catalyst and in crucibles and refractories and optical glass