Black amorphous precipitates of unknown composition have been obtained from aqueous solutions of sodium tetrahydridoborate and nickel salts. These precipitates have been analyzed by means of X-ray diffraction, electron microscopy, and electron energy loss spectroscopy, especially focussing on the energy-loss near-edge structure. By comparing the fine structure of the BK ionization edge of the nano-scalic product with that of amorphous boron and crystalline nickel boride, we show that the primary precipitate contains a metal boride formed at room temperature. The precipitate exhibits catalytic activity, e.g. for arylations of arylhalogenides. When annealed, the primary product transforms into crystalline Ni3B. An earlier crystal structure determination of Ni3B was confirmed by Rietveld refinement of highly resolved synchrotron data.Graphic