The origin is not obvious. You say "too messy to be Venetian." But, the closest I can think of would be an Indian knock-off. These have been circulating for thirty-something years. However, Indian beads are too messy to properly copy a Venetian bead. Plus they also use avventurina glass. So, by comparison, your bead is too neat to be Indian. And it looks older than the Indian period of production.

I suppose Venetian is not unreasonable, though it's not a pattern I recall. (A lot of Venetian beads from 100 years ago were cranked out. And these are messy, compared to the same beads when more-carefully made.)

JDA.