Amber is formed by the fossilization of resin due to oxidation and polymerization. Resin is a natural part of plants often released as a defense mechanism. There is no single tree responsible for the formation of amber.
Contrary to popular belief, amber is not fossilized sap. It is resin, which is a semisolid amorphous organic substance. It has a structure based on linked isoprene C5H8 units. Plants secrete resin in their pockets and canals through their epithelial cells.
When insects and fungi feed on a tree, it secretes resin as a defense mechanism to trap the predatory life forms. Resin also has healing properties that treat wounded limbs.
Natural forest conditions cause the volatile terpenoid fractions in resins to evaporate. If the non-volatile terpenoid fractions are strong enough to withstand degradation and environmental conditions, the resin fossilizes. Factors that contribute to resin disintegration include sunlight, rain, biological extremes, fungi and bacteria.
High pressures and temperatures provoked by overlaying sediment first transform the resin into copal. Often, it becomes incorporated into sediment and soil. Over the course of millions of years, it changes into rock-like sandstone and shale.
According to Emporia State University, the majority of amber that has been discovered is from Cretaceous and Tertiary sedimentary rock. This means it is 30 to 90 million years old.