Lisner Teardrop Pendant

I found this teardrop pendant in my mother's jewelry box. I remember her wearing this when I was much younger. After careful inspection I saw the copyright symbol followed by LISNER inscribed on the back. My questions are; is there a way to find the age of the piece, there is a small yellow glass shape on the back side, was this something like a trademark he did to his pieces, and what metals did he use for his jewelry. Thanks