I never recommend a safe word that doesn't have an aural aspect to it. It's so easy to get caught up in a scene that I know I wouldn't be looking at someone's hands. For that reason, I'd much more highly recommend giving your submissive some car keys. They always jingle. If she wants out, she can jingle them. The sound of car keys is so out of place that it will make you stop what you're doing.
As always, keep other ball gag safety in mind. Don't gag someone with a cold or who is having issues breathing. Don't use metal (or honestly, plastic) gag since they can chip teeth. Preferably use rubber or silicone or something soft. Make sure to not keep the gag in too long since it can cause issues with the jaw - a bit of pain is normal after awhile, but when it starts getting more than "a bit", it's time to stop. For most newbies to gags, this is around 20 minutes at the most. Also, if you have a ball gag in mind and the person you're playing with is new to gags, I'd recommend you keep the "ball" under 1.5 inches in width. A lot of cheaper gags are made at 2 inches in width, but most people can't get that much in their mouth, and that'd be disappointing to your play. If you don't already have a gag, I recommend the Silicone Ball Gag or the Tantus Beginner Ball Gag here on EF.
Happy gagging.
