What Were Antique Mourning Pendants Made Of?

by CAROL CLEMENT
(UK)

I have this antique mourning pendant which contains a lock of hair. It is 2 and a half inches long, including clasp, x 1 and a quarter inches wide. On the silver rim it says - " Memory 26 January 1879." On the silver initials on the back are markings but they are too small to read. How do I know if it is made of jet or glass.