Original icon, and 2 suggested alternatives
The "Insert reference" icon is not very intuitive. I've made a sketch of two alternatives.
See attachment, or on-wiki at http://commons.wikimedia.org/wiki/File:VisualEditor_-_Toolbar_-_Reference-edit1.png
That image includes the current icon, and two adaptations that consist of: a snippet of grey text, and a blue super-scripted number .
The grey text could be made into abstract letterforms - squiggles - (rather than the letters REF) in order to make it usable by all languages.
Or we could use a grey "+" sign with the blue [number].
(The only problem is RightToLeft languages. Not sure how to solve that.)
The blue super-scripted [number] is universally recognizable and intuitively understandable as denoting references, both in Wikimedia projects and even beyond. I'd suggest this element, at the least, should form the basis for any replacement icon.