The best way to determine whether your stain is metal or organic is by looking at its color. Different color combinations can help you identify what substance is causing a given stain.
Green and Brown: Organic – leaves or other plant material
Red and Blue: Organic – berries
Combination of Blue, Green, and Black: Metal – copper
Combination of Green, Brown, and Red: Metal – iron
Combination of Brown, Black, and Purple: Metal – manganese