Italian Bib Necklace

This was given to me nearly fifteen years ago. I know very little about it, except that it is Italian. Until very recently I thought it to be my Grandmother's as it was given to me by my Aunt, her daughter. When I showed it to my mother she looked confused and said she had never once seen my Grandmother wear the necklace. Sevreal months ago I took it to a jeweler out of curiosity. He marveled at it and asked if I was willing to part with it. I explained that it was an heirloom of sorts and that I was trying to figure out its origin and possibly its owner. I am hoping someone might be able to offer some insight.