Avatar: The Last Airbender / The Legend Of Korra

[Dracorexion]

They can't bend solid metal, they bend the imperfections in the metal. It's the reason why they can't bend pure platinum, meaning the same can be said for other metals. If someone made a pure iron bar, an earth bender probably couldn't bend it. Thinking on it now, glass is similar and different at the same time. While being a more pure form of certain elements in the earth, it's contains traces of limestone, soda ash, etc, making it about as impure as probably most metal that Earth benders are able to bend. However, glass is a lot less malleable than metal and earth, seeing as it cannot be bent or condensed without fracturing and becoming...well many small shards of glass.

Hm... I'm not sure whether it can be considered bendable or not. The several sub-sections of bending such as lava, lightning, blood, and metal all seem to have a certain requirements to be bent (bended?). Blood can only be bent when water is present within it and metal needs imperfections.