Not sure what material they use on that particuliar ship but rest assured that it is not see-through material. Also keep in mind that no balcony is all that private as those on either side of you can if they choose look around it and usually those above you can see onto your balcony and I have seen some that have spaces wide enough in the partitions where you can see your nieghbor either directly or reflected in the glass railing.
Just got off the Glory this past saturday. The partitions seemed to be a fiberglas type material. I didn't examine them in detail but can tell you they are not metal and as Jim advises, there are gaps, etc.and the shape of the partitions are such that you neighbors can easily peer around into yours, or you into theirs.
Balconies are nice but not necessarily all that private.