Further clarification:
Everything is buyable with eggs, but if you Don't have _enough_ eggs,
the menu gives the option to 'make up the difference' with Diamonds, as shmike says: "you Buy More Eggs, not goods".
The menu doesn't quite have an either/or option choice.
It will always use your eggs first, until they are used up,
and then use Diamonds; Either to make up the difference (if you have eggs) or exclusively (if you have No eggs).
Unless there is something I haven't discovered yet:
It isn't possible to choose to use exclusively diamonds if you have any Eggs left, It will always use up the remaining eggs First.
Contrariwise: If you don't want to use ANY diamonds: you just need to save up the requisite egg cost.