Formula: CaB₆, CAS No.: 12007-99-7. It is dark grey to black powder or bulk. The Vickers hardness is 9, density is 2.33 g/cm³. The melting point is at 2,200℃. It is stable at high temperatures in air and not soluble in HCl, Hydrofluoric acid and diluted H₂SO₄. It does not react with water, but it can be etched by strong oxidant such as Cl, F, HNO₃ and H₂O₂. The reaction with alkali is slow.