I haven’t seen this happening in Korra. Maybe you should ask a blog more specialized on Avatar. But no one can control wood either, so I’m not sure if all material are bendable in a way.

As glass is supposedly fluid, maybe even waterbenders can control it? Hmm. Given that metals have a very different chemical constitution than usual earth, I’m not sure the “metal is purified earth” explanation makes that much sense. It depends on the authors :-)

Other than someone figuring out platinumbending (which would make them OP, so I’d prefer not), I’ve been wondering about glassbending as a subset. Glass is made from super heated sand, which we already have precedent for with the sandbenders. I don’t know if glass is too much of a stretch for bending, with the chemical reactions vs physical reactions used in making glass. But I think that could be an interesting direction. Can earthbenders make glass, but once it’s in its glass state they can’t anymore? If an earthbender was to fix a broken window, could they do it with normal glassbending, or would you need to be a lavabender to heat the glass to the point where you can meld pieces together? (If one lacks lavabending, have a firebender on hand to help with the heat). 

Bending Light. 

...possibly?  One would think that glass would have different properties to explore than lava, even if it is made from superheating sand -- a lavabender might be able to create glass, but bending it once it’s already created depends on whether or not the end product still counts as earth.

I don't think earthbending depends on either of those things, actually.  It can't be silica, not only because earthbenders can bend metal but also because they can bend coal.  Basing it on crystalline structure doesn't make much sense, either, because there are some crystalline solids that earthbenders definitely can't bend like ice and sugar.