Amber (or, technically, resinite) is fossilized tree resin (not sap), which has been appreciated for its color and natural beauty since neolithic times. Good quality amber is used for the manufacture of ornamental objects and jewelry. There are five classes of amber, defined on the basis of their chemical constituents.

Because it originates as a soft, sticky tree resin, amber sometimes includes animal and plant material as inclusions.

About amber : http://en.wikipedia.org/wiki/Amber