Got 5 diamonds as well, but found it too boring once I got a streak of 20+ without an unlocked one.
I spend the diamonds on production slots, so you can keep most machines running during the night. Helped my sugar stock a lot!
Other than that you could always buy Tom for a day and get jewlery for example from him.
edit: I try to always have at least 15 diamonds in case I should want Tom (hardly happens, but who knows...).
And I also try not to have too many, because I don't want to accidently spend them.