Board Thread:Naruto Discussions/@comment-25369463-20140908104317/@comment-4912646-20140910033907

Problem is, there are only four possible combinations of natures left. Two of them are Magnet and Explosion, and the third one might be Blaze (though I personally suspect that's Fire + Yin). So unless it is possible to combine the same two natures and get different results (for example, Water + Fire being both Boil and Bases), coral, steam, bases, ink, and scale powder can't be their own separate natures (let alone "negative emotions").

All that aside, if Isobu had Ice/Crystal/Coral Release, why wouldn't it have added it into the Rasenshuriken instead of generic water? If anything, creating coral should be an application of Yang Release.