What is Shanks' sword made of? At Marineford, Shanks stopped Akainu with his sword. That much we all saw, but what I noticed was that he did not use Haki on the sword and yet it did  not melt.

This has been bugging me. How is this  possible, if Akainu's Lava can burn even fire but not melt the sword which is not even infused with Haki.
The black coloring induced by Haki was only introduced after the timeskip. Before that, when Haki is used, it is not visible with two exceptions: Garp's punch at Enies Lobby and Whitebeard's fist at Marineford. 

For example, in chapter 512, Rayleigh faces Kizaru in a sword duel. Rayleigh is using a simple sword while the admiral is wielding an energy sword made of pure light. In this fight, for Rayleigh's sword to be effective against Kizaru, he is obviously using Haki, even if the Haki is not visible at all. This is confirmed by the fact that Rayleigh's sword swing injures Kizaru, a logia user. 

Therefore, Shanks' sword is not "made" of anything special (i.e. seastone), he was simply embedding it with Haki. He may be wielding one of the world's great swords but this has yet to be confirmed.