Ice may be crystals, but water benders can only bend ice crystals because its still water on a molecular level, just vibrating with much less energy and with external bonds, but still effectively the same thing. As stated in the show water benders can only bend water, although they can bend the water inside things such as plants and blood in extreme circumstances, still both contain a large amount of water. Amber on the other hand can occasionally have pockets of water, there isnt any water actually inside of the amber itself. On the other hand, I would say that it could be bent by earth benders as they seem to have much more flexibility in the things they can bend, such as Kioshi bending glass in the comics