The mandrel (or rod through the center which makes the hole and also used to "mount/hold" the bead in the heat) is usually made from iron as it cools down faster than the glass, allowing it to be removed as the glass starts to cool and settle in shape - this forms the hole through the bead. As a result of the release (removal) of the rod, the oxidised iron leaves a deposit of iron oxide on the inside of the bead hole. a grey residue, within the hole. This can mostly be washed off, or will disintigrate away gradually.
