Ok, so midnight stones have been discussed to death but I'm still completely in the dark about elementium and destiny's sliver.
The latter appears to be nothing more than vendor chow to the tune of 250g per pop. Nice but more than a bit opaque since it appears to be questy in some way.
Elementium is curious. I was able to buy things requiring it for absurd prices, such as 6250g for the "Karrazan's Sword" (World of Warcraft much? Mixing BWL and BC in the same item is just too funny) but said price PLUMMETED as soon as I found my first cache of elementium. To the tune of 2200g which is a bit over a 4k drop in price.