Metal beads are made of zinc alloy, which is free of nickel and lead. Apart from zinc, the other major ingredients of the alloy are:
Aluminum: to make the alloy softer for shaping, and produce smooth surface of the products.
Copper: to keep the alloy products hard enough for daily abrasion.
Magnesium: to increase hardness of the alloy products to stand daily abrasion.
As zinc alloy metal is much cheaper than 925 sterling silver, it's an ideal substutite material to make beads for fashion jewelry.