Sirce Luckwielder (paraphrased)
So each anti-investiture is like its world's investiture, but can't be effected by it. So aluminum can't be affected and destroys Allomancy, ralkalest can't be Soulstamped, Shardblades are blunted by that one thing. Is the black filled sphere that Galivar gives to Szeth the anti-investiture to Stormlight?
Brandon Sanderson (paraphrased)
When I asked this, he became much more tight-lipped and said that was an interesting theory and that I would learn more in book three.