Second-hand synthetic diamonds, sure - but buying anything second hand is generally more morally defensible than buying that thing new.
Buying a used natural diamond is still worse than buying a synthetic one, because you're propping up the resale value of natural diamonds and therefore encouraging other people to buy them.
Buying a used natural diamond is still worse than buying a synthetic one, because you're propping up the resale value of natural diamonds and therefore encouraging other people to buy them.