Copper(I) oxide is found as the mineral cuprite in some red-colored rocks. When it is exposed to oxygen, copper will naturally oxidize to copper(I) oxide, but this takes extensive periods of time. Artificial formation is usually accomplished at high temperature or at high oxygen pressure. With further heating, copper(I) oxide will form copper(II) oxide. Although copper is clearly essential for a wide range of biochemical processes which are necessary for the maintenance of good health, copper is also a potentially toxic substance The richest dietary sources of copper include nuts, seeds, legumes, the bran and germ portions of grains, liver, kidneys, shellfish, oysters and crustaceans. Cow's milk has little copper.