In very simple terms, in order to qualify a piece of resin as true Amber, a couple of things need to have happened. First, the molecules must have formed a polymer (a compound where two or more molecules have joined together) and secondly, the specimen must be at least 100,000 years old.
Copal or Copalite is the term given to organic resins that are not old enough (i.e. younger than 100,000 years) to have fossilized and hardened sufficiently to become Amber.
Amber is often imitated by plastics, colored glasses and some modern tree resins. True Ambers have a low specific gravity (amber can float on salt water) and inclusions can distinguish it from plastics and glasses.