Amber is a hard, yellowish-brown fossilized resin. It comes chiefly from the resins of pine trees that grew in northern Europe millions of years ago. These resins were gummy materials mixed with oils in the trees. When the oils became oxidized (combined with oxygen), hard resins were left. These pine trees were buried underground or underwater, and the resins slowly changed into irregularly shaped lumps of amber. Lumps of amber often contain insects trapped as the resins flowed from the trees. Some lumps have air bubbles.