Lanthanum Boride, also called Lanthanum Hexaboride or LaB₆, is an inorganic chemical and refractory ceramic material. It has a high melting point of 2210 °C and is insoluble in water and hydrochloric acid. It is extremely hard, with a Mohs hardness of 9.5. It has a low work function and one of the highest electron emissivities known, and is stable in vacuum. Stoichiometric samples are colored intense purple-violet, while boron-rich ones (above LaB₆.07) are blue. Ion bombardment changes its color from purple to emerald green. LaB₆ is a superconductor with a relatively low transition temperature of 0.45 K.