[Pursuivant]

With an Energy Cost of 30 to cast, Rebuild is effectively a form of Quick-and-Dirty enchantment, albeit with quicker results. Other than that, Rebuild could indeed do the job.

There's nothing in GURPS 3E Grimoire (where the spell originally appeared) or GURPS 4E Magic about needing a minimum amount of mass from the original object in order to cast the spell and nothing to imply that "raw materials" can't be created or repaired. The text is virtually identical in both cases.

In both Grimoire and 4E Magic, it's specifically stated that you could rebuild an entire starship from just a portion of a bulkhead (as long as you cast Schematic first) and have sufficient energy to replace the missing mass. That implies that as long as you have even one atom of the original item left, you could rebuild it.

Both also say that there are no TL penalties to rebuild objects simpler than machines. GM's choice as to whether attempting to recreate a diamond counts as "exotic material."