One Piece Episode 604-605

[Assertn]

Do you remember a scene where one of the CP9 tried shigan against Luffy and it didn't work? That's what Assertn was asking and I didn't remember either. That would naturally immediately tell it's not haki, because haki negates totally any fruit power effects. If you can tell me the ep and minute with such a scene, I will admit full defeat. If shigan is not haki, I won't bother to defend tekkai either. Then it's just one more example of how muscle is tougher than steel or explosives in the shounen universe (makes Franky look kind of stupid if a normal human body is tougher than a cyborg body, though).

I don't remember any specific occurence, but according to this: http://onepiece.wikia.com/wiki/Rokushiki#Shigan
the shape of the finger makes a difference between whether or not it is able to pierce luffy.

Also, I don't know where they got this from, but they also say this:
Tekkai is similar to Busoshoku Haki, as both reinforce the users body in some way, the difference being that Haki is based on the user's willpower and Tekkai is based on the user's physical power.