The LPF test masses are gold-coated cubes made from an Au-Pt alloy, with a side length of 46 mm, and a total mass of 1.97 kg.  The material has been chosen first for its magnetic purity – this alloy is tuned to null the magnetic susceptibility that gives an induced dipole moment and allows near elimination of contaminants that give a permanent magnetic moment – and for its density, maximizing inertia to give the smallest possible accelerations for a given stray force.  The entire TM is coated with gold, to ensure an homogeneous equipotential surface, that also serves as a mirror for the interferometry readout.