If your girlfriend is new to butt plugs, I would not recommend that she starts with glass butt plugs. These are really geared more for people who are experienced at booty play because glass doesn't stretch. There's no give to it, and it's not soft.
One thing I do love about glass butt plugs is that they are the smoothest surface you can get. There's no friction whatsoever. They're also hygienic which is great for anal play. They're easy to clean because you can sterilize them in boiling water.
The type of glass they are made from is not easily broken. So, for the intended use, possible breakage isn't something that you should worry about.
