Complex sentence: Polymerized single walled nanotubes (P-SWNT) are one class of fullerites which are very hard like diamond, but because the nanotubes intertwine they don't have the corresponding crystal lattice that makes it possible to cut diamonds neatly.
Split into simpler sentences: Polymerized single walled nanotubes (P-SWNT) are a class of fullerites and are comparible to diamond in terms of hardness. However, due to the way that nanotubes intertwine, P-SWNTs don't have the corresponding crystal lattice that makes it possible to cut diamonds neatly.