I was just thinking about Valyrian steel and its properties. What makes it so sharp/hard/light when this idea hit me. I believe valyrian steel is a combination of traditional steel and obsidian, also known as dragon glass. I have several reasons for believing this.
1. Valyrian steel is famous for among other things its sharpness. I think it is accurate to say that valyrian steel swords are sharper regular steel swords. And what material is known for its sharpness as well, obsidian. Examples of this include the maesters at the citadel cutting themselves on the obsidian candles, several people cutting themselves on the obsidian daggers Jon found, Jon himself proclaiming that Obsidian was sharper than steel, but brittle, and several other examples.
2. Obsidian is some form of volcanic glass (made by volcanos or perhaps dragons), If this is correct, then Obsidian is in fact harder than steel (to be clear I am not saying it is not brittle but by the scientific definition of "hardness" obsidian is certainly harder than steel). This could explain why less steel is necessary for the forging of a Valyrian blade.
3. Obsidian is lighter than steel, which could explain why valyrian steel is lighter than traditional steel.
4. In AFFC, Sam claims that "Dragonsteel" (which I believe is Valyrain steel) is effective against the others. The only material that we know of that is effective against the others is obsidian. Therefore it would make sense that obsidian is part of the construction of Valyrian steel.
5. The fifth reason I have for believing this is the way that valyian steel acts with coloring. From ASOS Tyrion IV:
"Nor I, my lord," said the armorer. "I confess, these colors were not what I intended, and I do not know that I could duplicate them. Your lord father had asked for the crimson of your House, and it was that color I set out to infuse into the metal. But Valyrian steel is stubborn. These old swords remember, it is said, and they do not change easily. I worked half a hundred spells and brightened the red time and time again, but always the color would darken, as if the blade was drinking the sun from it. And some folds would not take the red at all, as you can see. If my lords of Lannister are displeased, I will of course try again, as many times as you should require, but—"