The centerpiece of this necklace is a Bactrian seal bead made of steatite dating to circa 1,000-200 BC.
Bactria is the name of an ancient country lying between the mountains of the Hindu Kush and the Amu Darya river in what is now part of Afghanistan, Uzbekistan, and Tajikistan. Bactria was especially important from about 600 BC to about 600 AD, serving for much of that time as a meeting place not only for overland trade between East and West, but also for the intermingling of religious and artistic ideas from neighboring cultures.
Seals were commonly used in many ancient cultures, from Neolithic times on, to identify property by pressing the incised surface of the seal into a daub of wet clay placed on some string or other cordage tied to secure the property. I suspect, however, that this bead was primarily used for adornment.
Strung with modern jasper, garnet and gold filled beads. The clasp is gold filled.