We have discovered massive icosahedral multiply twinned particles of boron carbide among soot produced during the arc evaporation of carbon and boron in the presence of magnesium. The observation is striking because 5-fold symmetry in large micron size crystals is extremely rare. It is suggested that the icosahedral structural unit that builds up the rhombohedral crystal structure of bulk B4C provides a template for the formation of these large icosahedral crystals. The physical properties of boron carbide, known for its hardness, could be further improved in these crystals because of their unusually high symmetry.