Amber consists of a fossilized tree resin and has been used for hundreds of years for decorating. It has also been used for jewelry since the Stone Age, about 13,000 years ago! Amber is formed due to "molecular polymerization whichresults from high pressure and temperature produced by overlying sediment, which, as Wikipedia explains, initially transforms the resin into copal and then "sustained heat and pressure drives off terpenes and results in the formation of amber".