This might be a mistake, or is a really stealthy reveal on Brandon's part, from the first page of Chapter 24:
“Marasi dug in her purse, pulling out the little spike that belonged to ReLuur. Such a small thing, and so clean-a shining sliver of…pewter, was it?”
Pewter is not one of the metals used in the four types of Blessings used in the Final Empire. So if this isn't just a "typo", then either Marasi is mistaken or the kandra have figured out how to create Blessings from spikes granting Feruchemcal abilities, which I thought Brandon had said they hadn't.