icon-calendar (U+F073) appears blank in Opera.
I don't know very much about the Unicode PUA and font-related issues so I can't be of much help here, but this person seems to have had a similar issue (http://stackoverflow.com/a/10748396) so maybe that information could help.
This issue affects all of the platforms tested:
Confirmed. Reached out to Opera dev relations to see if they know what could be causing this
So, this is indeed a known bug at Opera and should be this one: http://opera.r3m.co/bugs/index.php?action=bug&id=1328
Could we simply not use that code point for compatibility reasons? Sure, they'll fix it at some point but the older versions will exhibit the issue forever, obviously...
All icons are appearing blank for me in Opera Mobile 12.10 on Android 4.1.2 (Galaxy Nexus) and 4.2.1 (Nexus 7).
Sorry, I'm not going to be spending much time to specifically support Opera. Icon should work.