First off congratulations on doing so many reviews and so quickly! Steward badge in 7 months is quite something!
So long as you are doing good reviews, and not just hammering on the buttons to get the badge, then there is no reason at all that you should stop*.
If you want to keep reviewing, then keep reviewing!
(Top reviewers on SO have over 45K reviews in ...
There are 490 moderator across all the SE sites. (so you are bound to see a diamond or two.)
Yet only the three BSE mods (gandalf3, iKlsR, and me) and SE employees will have diamonds attached to our name on this site.
The three users you linked to are employees of SE (and have mod+ abilities on every site.)
Oded is a dev at SE.
Robert Cartaino is a ...
According to Meta.SE, the criteria is:
(total questions - negative questions - closed - deleted)/total questions >= 0.5
where each question can be counted more than once (i.e. a closed deleted negative question will counted as -3 total).
So any question with a positive score (that has not been closed or deleted) is considered well-received.