Nickel Carbonate is light green and paramagnetic, and will not retain magnetism. Stainless steels are typically 15-20% Nickel.
Didn't Ogfusionist, ~3 years back make some considerable case for nickel carbonate in his reported observations. Or was that my own deduction based on his description? Briefly, Ogfusionist had observed that FibreFrax containing some nickel of a particular oxidation state, when raised to a particular temperature in the presence of dry hydrogen, led to a spontaneous thermal runaway that melted or otherwise fused the fibrefrax substrate.