Based on the hypothesis that this H-induced crystallization mechanism is general, and may be operative in other covalently-bonded group IV solids, we have exposed multiwall carbon nanotubes (MWCNTs) to H atoms produced by electron impact dissociation of H2 molecules in an inductively coupled radio-frequency (rf) plasma. We hypothesized that reactions with H atoms like those observed in silicon may help bind carbon atoms in neighbouring concentric CNTs and transform them into diamond nanocrystals. Indeed, we observed hydrogen-induced transformation of MWCNTs to various crystalline carbon structures, even at room temperature. Specifically, exposure of multiwall carbon nanotubes (MWCNTs) or multiwall carbon nanofibres to atomic hydrogen transformed them into other carbon allotropes including cubic diamond and hexagonal diamond (lonsdaleite). Nanometer-size crystals of diamond appear gradually in webs of long strings of crystallites, ~ 5-50 nm in diameter, where nanotubes once laid. This H-induced transformation of MWCNTs to diamond is observed even at room temperature. Moreover, high-resolution transmission electron microscopy of the H-exposed material reveals the presence of carbon nanocrystals whose electron diffraction patterns and lattice spacings could not be accounted for by known crystalline phases of carbon, such as diamond and lonsdaleite, or by contaminants. In addition to cubic and hexagonal diamond, interactions of H atoms with the concentric graphene layers of the MWCNTs produce new crystalline carbon phases that have not been observed previously. Specifically, we show, unambiguously, the existence of a face-centered cubic carbon phase with lattice constant a=0.426 nm.