Mark Zuckerberg's Wife Priscilla Chan-Zuckerberg -- The SIMPLE Ruby Engagement Ring

If Mark Zuckerberg's new wife was expecting a MASSIVE diamond on her wedding ring ... she was SORELY disappointed.

Just days after the surprise wedding of the century, Priscilla Chan-Zuckerberg took her brand new ruby rock on the road ... flashing her piece during a lunch date with a friend in Palo Alto, CA yesterday.

According to reports, the $20 BILLION Facebook honcho -- who has a reputation for not caring about excessive displays of wealth -- designed the ring himself.

So, we gotta ask ...