In my EasyTutu game, the Greenstone amulet is not protecting against the stun caused by the 5th level version of Chromatic Orb. (I haven't tested to know whether or not it protects against other forms of stun.)
In Fixpacked BG2, the greenstone amulet does protect against it, and the description suggests that it should, so I think this is a bug.
I think I have the most current version of EasyTutu, but I'm not sure. (One thing that annoys me about EasyTutu is the lack of version numbers, or any convenient way to check.)
Other mods installed: Degreenifier, BG1NPC, BG1UB, 1PP, Level 1 NPCs, some stuff from G3 BG2 Tweaks, and SCS. I don't think the mods are responsible, however, since I get the same effect on another install with different mods installed. (Including no BG1UB or SCS, which would be the most likely culprits, I think.)
Anyway, I've no lost two solo "No Reload" attempts in chapter 5 on top of the Iron Throne from this, so it's a sore subject. (Darn you Naaman!)