A novel model system of hard colloidal platelets was observed to phase-separate into an isotropic and a liquid crystalline phase. Polarization microscopy revealed that the liquid crystalline phase was of nematic origin. With such orientational ordering in suspensions of platelike particles already being predicted in the 1940s by Onsager＇s theory and corroborated more recently by computer simulations, a direct comparison with experimental observations has now become possible. Furthermore, the apparently unhindered formation of a macroscopic nematic phase in a suspension of hard platelets sheds new light on the issue of gelation instead of nematic phase formation in suspensions of clay platelets.