Amber is fossil tree resin. Although not mineralized, it is often classified as a gemstone. A common misconception is that amber is made of tree sap; it is not. Sap is the fluid that circulates through a plant's vascular system, while resin is the semi-solid amorphous organic substance secreted in pockets and canals through epithelial cells of the plant. Because it used to be soft and sticky tree resin, amber can sometimes contain insects and even small vertebrates.
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Fossil Eocene bird 48 million years old | (from Flickr)