I don’t own a moleskine so I can’t verify it, but Joe from engraveyourbook.com claims that he had a ‘organic chemistry test’ done on a piece of the cover that informed him that it was composed of vinyl (PVC).
PVC fumes are highly toxic to breathe in and are also damaging to laser engravers’ innards. So if you are in the business of laser-engraving them, you may want to hold off until the safety (or toxicity) is verified.
Note that having a laser-engraved moleskine is probably not so bad, just wash it off to make sure there’s no dust left.
[note: if you have a moleskine, you can try 'pulling' a corner. If it stretches, its almost certainly vinyl.]