Amber is fossilized tree resin, it is not produced from tree sap. It has formed from resin that oozed out of the cracks in the bark of trees millions of years ago. Sometimes the sticky resin captured insects and other organic matter. This aromatic resin can drip from and ooze down trees, as well as fill internal fissures, trapping debris such as seeds, leaves, feathers and insects. The resin Left to harden and then becomes buried in sediments the amber endured and fossilized through a natural polymerization of the original organic compounds.
What is resin?
Resin protects plants. The resin may be produced to protect the tree from disease and injury inflicted by insects and fungi. it has antibacterial properties and the sticky substance hinders insects gnawing or burrowing into bark. Some trees can produce resin in such large quantities that it seeps out of cracks in the bark and runs down their trunks. Resin may be exuded to heal a wound such as a broken branch, and resins possess odors or tastes that both attract and repel insects (Langenheim, 1969, p. 1167). In mature trees, resin may simply exude from vertical fissures in the bark due to tension produced by rapid growth (Langenheim, 1969, p. 1166). Resin may also be produced as a plants method for disposing of excess acetate.
How is amber formed?
Amber is a fossilized resin, not tree sap. Sap is the fluid that circulates through a plants vascular system, while resin is the semi-solid amorphous organic substance secreted in pockets and canals through epithelial cells of the plant. Land plant resins are complex mixtures of mono-, sesqui-, di-, and triterpenoids, which have structures based on linked isoprene C5H8 units (Langenheim, 1969, p. 1157). Volatile terpenoid fractions in resins evaporate and dissipate under natural forest conditions, leaving nonvolatile terpenoid fractions to become fossilized if they are stable enough to withstand degradation and depositional conditions. The fossil resin becomes incorporated into sediments and soils, which over millions of years change into rock such as shale and sandstone
Resin first hardens into a substance called copal and chunks of copal get buried in soil and sediments. Over millions of years the copal hardens into amber. Any insects or other inclusions that were stuck in the resin are perfectly preserved.
Individual amber pieces often contain evidence about how they were formed. They can contain pieces or impressions of bark, or many layers that built up by successive flows of resin. Both copal and amber are light so can be carried by water far from where they originally formed.
Therefore, amber is formed as a result of the fossilization of resin that that takes millions of years and involves a progressive oxidation and polymerization of the original organic compounds, oxygenated hydrocarbons