black cubes in block water jackets...block catalyst?

[87hilux7mge]

so, when i was putting the cyl head on the motor the other day, i was inspecting the water jackets (damn my ocd cleanliness) and saw two 1.5 inch sized, black cubes that look like coal or charcoal, and i was concerned as to what they were.

my thought was a block catalyst of sorts, to prevent corrosion. i just want to make sure its not something foreign, that shouldnt be in there.

any thoughts? i know, i should have taken pics, but i forgot so