We recently got this intriguing passage.
Conjecture implies that this silvery metal is aluminum. However the other property of the spear, stormlight draining, implies that some sort of fabrail is at work here. This is a conundrum as aluminum would prevent any fabrail from working.
Somebody suggested to me that maybe the spears just have sides coated with this but I don't think that works. Kaladin and his Windrunners would have long since grown proficient at severing the spearhead and leaving the fused with an aluminum stick. This leads me to the belief that the aluminum is actually part of the fabrail and that has certain implications for how it works.
We know that aluminum resists all forms of investiture. I am fairly confident that Kaladin will never manage to lash an ingot of aluminum. We see various uses for this property in many shardworlds(particularly on scadrail). The question that has bugged me for a while is why? How is it that aluminum does this. I think I have an answer.
Think of kinetic investiture like electrical charge. Things that are already invested are much harder to effect with investiture. Kaladin actually notes something like this in this very chapter when thinking about lashing his Sylspear.
Similarly you can't really overcharge a battery(without a lot of power and then it tends to blow up) and if you break a circuit it stops working as electrons don't want to be concentrated in too small an area.
Taking the similarity one stop further what if aluminum is a conductor of investiture? This would explain how they manage to drain stormlight out of a radiant. Connect a gemstone to some aluminum and the stone will pull the investiture out of the radiant and into the stone. Cut the stone right to make sure it leaks easily and you can expend most of radian'ts power into the air. This would also explain scadrail spoilers.
People who are good at undergraduate physics and everyone else please correct my mistakes.