Crystalline particles of palladium are known to form on polycrystalline Pd interacting with low-energy hydrogen ions. These particles disperse on the glassy medium called the “matrix". The particles were recently confirmed by transmission electron microscopy to be classified into two groups: the particles emerging from the projectile-implanted subsurface together with the outflowing matrix and those newly produced on the hydrogen-bombarded matrix. The latter type of particles was nucleated as a crystalline cluster on the disordered substrate, and then underwent three-dimensional growth into a nanocrystal under the bombard- ment of showering hydrogen ions. Some particles presented a bubble-like TEM contrast, independently of their growth history. Such particles were chestnut-like in structure, with a hard shell wrapping the less-dense interior, and their formation may be attributed to a chemical process occurring within the particles.