A process for preparing a powder of a material of natural origin, which includes a step of treating a substance containing at least 70% by weight of one or more leather wastes in particle form in a twin-screw extruder, wherein a temperature profile is applied that increases from an initial temperature of between 10 and 40 C. up to a maximum temperature of between 60 and 120 C., in the presence of water in an amount such that the weight ratio of water to substance is between 0.20 and 6.