While "treasure hunting" one day we came across an interesting bottle that was violet in color and looked like a window that was crystallized from the cold.
We asked questions and were told that these bottles had spent time in the sun; they had graduated from clear to amethyst, because they had manganese in them. Manganese is a metal chemical (Mn) atomic #25, and often found in combination with iron. Depending on the oxidation state there are several different pigment ions (colors) that can appear.
Manganese is used to prevent rust and corrosion on steel. It is used in the stainless steels. It turns out some awesomely colored bottles.
Be on the lookout for these beautiful bottles!