Spalting is a by-product of the rotting process that is carried out by a vast array of stain, mold and decay fungi, that are found naturally on the forest floor. When the temperature and humidity are right, spalting can cause many different and beautiful patterns in rotting wood. The unusual colouration may be due to chemically induced reactions between the wood, fungi and insect deposits, often resulting in black, pink, grey and multicoloured streaked wood.
I have seen it used when the wood has been stabilized.
Also seen it used when the wood hasn't been stabilized. That would worry me