The material is in doubt. I think it might be glass or even faience. If it is in the quartz family it's more likely to be quartz than agate. And, if it is quartz it is very possible the bead was glazed. Glaze being glass, the bead would then have a glassy aspect—that I think I can see in your photo. Plus, there are places where it appears parts may be absent. This could happen to the bead, whether it is quartz, faience, or glass.
Can you say how the bead was determined to be agate?
Jamey