That is a great question. While setting up the 20 gallon one I put rocks on top of them and they are holding up really nice. I would say they are sturdy to an extent.
The tips and any thin areas will be easy to break off or chip. Any hard hits like dropping a rock from over a foot above it onto the glass will most likely crack it. It is hard thick glass so it has a lot of weight behind it to keep its structural integrity, but it is glass.
Basically I think its sturdy enough to take weight and pressure but not to be thrown or anything like that.