Vintage Gold Bead, Rare Venetian Beads, Filigree Lacey Gold Bead, One of a Kind, Jewelry Supply, Craft Supply, Stitchery Supply
I don't know mm measurements but this is .5" as you can see in photo next to ruler. It's very lightweight but I believe it is genuine gold or gold plated. (Old Venice, Italy craftsman don't waste their talent and time on fake stuff.) The center hole for threading is small and I can't tell you what size that is either.
This bead came from a very high end necklace of Venetian glass beads. That necklace was purchased in the 1980s but this bead is quite possibly much older that was incorporated into the necklace.