People often ask which is better, glass or acrylic domes. Although I do prefer glass, my answer is a typical "it depends", as there are tradeoffs between the two. One of the tradeoffs has to do how the dome deals with getting bumped by things that can bump dome ports... things like dive ladders, rock, coral, shark teeth, and other camera housings. These things can of course leave scratches in your dome port, which no one wants. Another way scratches can happen to domes is grit - a piece of grit in your cleaning cloth, some sand caught in your neoprene dome port cover, and even the skin of a shark - those are ways that grit can scratch your port.
The conventional wisdom is that acrylic ports are quite a bit easier to scratch, but scratched acrylic can be buffed out with some polish and some elbow grease. Glass ports are much more resistant to scratching but much more difficult if not impossible to fix.
I've been using one of the of Zen 100mm dome ports for my Tokina 10-17; in fact it is a prototype version of the port and even features a non-anodized section that is holding up surprisingly well. I mention this because I've been using this port for quite a while now, and loaning it out to people, and up until last week, not a single scratch existed on that beautiful coated glass finish. Up until last week. Now I'd like to say that I didn't scratch the dome myself. I'd like to believe that someone I lent it to, or one of the divemasters who handle the rig, did it, and in fact I do not remember bumping it in such a way that would put such a nasty gash on that glass, but I can't rule it out. In any case, there it was, nearly in the center of the dome, a little series of about 6 abrasions about a millimeter or two across. Scratches were bad enough in the good old days, but now that we shoot video with these housings too, it is worse. I actually wanted to use some footage I had shot with the dome in this state, and I can tell you it is a PITA to hide that in video.
So, head hung in shame, I showed the dome to our mighty Nauticam USA repair technician, Shen. (Shen is not only a wizard of repair, he's also a wonderful u/w photographer - he just won a first prize in the Our World Underwater Photo Contest). He gave me an appropriately difficult time about it, and before I could even protest my innocence, he started in working on it. Zooming back to that "conventional wisdom", I was prepared for him to replace the glass. Now, I did take High School Geology, and I do remember the hardness scale. It turns out that aluminum is a lowly lower third on that scale, and quality glass is in the upper third, and that alleged scratch on my dome port? Not actually a scratch - but in reality a lump of material deposited there when the hard glass came into contact with the softer metal. Shen removed the material and polished the spot, and now my dome is as good as new. Holding it up in the light, I cannot see where the "scratch" was. Perfect.
So the moral of the story? Don't dive on a boat with a titanium dive ladder, and if you do scratch your dome, it might not be the end of it, even if the conventional wisdom says otherwise.