Turquoise and Black Necklace Firm

I purchased this black & turquoise necklace at a boutique shop in Marblehead, MA. years ago, when we lived there. There was a jewelry artist who made it. The clasp is marked, I think it says 928 or 925. Hard to read. The clasp has tarnish. Please zoom in as part of description. I forgot what the lady told me the black stone/beads were.