Well, she also wore her very real new diamond earrings when watching sports at the Olympics...
...but this piece here is certainly fake. You can tell by the setting and by the look of the outer cluster of 'diamonds' - they look like glass on an old 1950s vintage piece, which this could actually be.
A bit out of character for MM - most of the time, her jewels are real, and whenever she wears costume jewelry for 'fun', it tends to be bold and easily recognizable (just like costume jewelry should, IMO) instead of faking the real thing.