I would venture to guess that you could search your ENTIRE LIFE and never find another necklace like this.  I've been in the antique/collectible jewelry business for over forty years and I can tell you that it's the ONLY ONE I've ever seen.  With that said, (and to be crystal clear, so there are  no misunderstandings) this appears to be a contemporary artisan piece that was made using antique abalone buttons.  The buttons are absolutely 100% antique, and judging by the clasp, chain and bezel setting of the buttons, the necklace was made sometime in the past 20-30 years.  The length of the necklace is approximately 18" with the center button measuring 1 3/8" across and the other four buttons measuring 1 1/8".  It's a LARGE, MAGNIFICENT looking piece when seen in person (If ever there was "Statement Necklace", this is it...) 
  Weight on this piece is a whopping 48.0 grams.