The liquid crystal phase behaviour of suspensions of charged gibbsite (Al(OH)3) platelets is investigated. We study the competition between sedimentation, gelation and liquid crystal formation at different initial concentrations and salt strengths. By variation of the ionic strength we are able to tune the effective thickness-to-diameter ratio of the platelets in suspension. This enabled us to experimentally test the liquid crystal phase transition scenario that was first predicted a decade ago by computer simulations for hard platelets, that is the isotropic (I) nematic (N) and isotropic to columnar (C) phase transitions in one colloidal suspension. In addition to the shape-dependent thermodynamic driving force, the effect of gravity is important. For example a biphasic (I-N) suspension becomestriphasic (I-N-C) on prolonged standing. This effect is described by a simple osmotic compression model. Finally we discuss the possibility to produce photonic crystals from the iridescent columnar phases of Gibbsite nanoparticles.