Turns out - nickel carbonate is not only green but also -
brown. 
Physical properties
NiCO3: Light green rhombohedral crystals; decomposes on heating; practically insoluble in water, 93 mg/L at 25°C; dissolves in acids.
2NiCO3?3Ni(OH)2?4H2O: Light green crystals or brown powder; decomposes on heating; insoluble in water; decomposes in hot water; soluble in acids and in ammonium salts solutions.
Zaratite: Emerald greed cubic crystals; density 2.6 g/cm3; insoluble in water; soluble in ammonia and dilute acids.
LINK - https://www.chemicalbook.com/ChemicalProductProperty_EN_CB1374836.htm
Driftwood has lots of brown.
Thing is... if bulk tonnage was presifted - removing impurities - prior...
And if assaying by MGX ( most recent ) did not search for nickel...
Then,. one won't know of nickel.
Nickel in a brown carbonate form - is indeed an eye opener.
Decomposition color change.
One could easily pass it off as simple - brown dolomite.
But is it ?
Cheers.....