You go to the "Casting Metals" tab (which makes no sense for glass¹) and select any metal - doesn't matter what - then scroll all the way down. Well beyond the point you've given up in despair.
I'm not against computer programmers - I was one myself, once, many years ago - but they do seem to try to prove that they never, ever, use the program they've written and never, ever, do anything even remotely related to the activity they are programming a widget to make "easier" (don't get me started on watches and gps gear for tracking running, cycling or swimming (no, I'm really not a triathlete, but I've done a couple)).
I'm sure the Jagex team are totally lovely and really well-intentioned. But something is going horribly wrong. And--no, really--I'm not one of the "it's all going to die" brigade: I think they have a great product and a really interesting game. But, sometimes, some stuff is so left field it smacks you on the right side of your head. You have to assume that the guys and gals programming Runescape don't play it. If they did, they'd all be doing lemming impressions.
¹ the only way glass makes sense as a metal is if you are an astrophysicist: they think every element after helium is a 'metal'. Bless.