Spikes used in kandra are, by definition, metal. I can see soulstone being the same power manifested in a different solid, in this case rock (which would make sense as Devotion and Dominion are heavily invested in the land itself) but, once again, I really doubt it's the same material as Paalm's spike.
Also, side note, I picture the spike being streaked with red, like rusty splotches, not particularly visually attractive, rather than soulstone, which Shai describes as being marbled and beautiful.