Amber fossils are the remains of ancient organisms trapped in amber, tree resin which has hardened through a process called polymerization. If the tree resin happens to harden in the right environment, it can endure for hundreds or thousands of years, creating a valuable record of organisms which lived in the Earth's past. Also known as fossil resin, amber has also been prized for thousands of years as an ornamental gemstone, though is it not a mineral, and thus not technically a gem.
The formation of amber fossils starts with the secretion of resin. Trees and some plants secrete resin as a protection from insects. Although this sticky secretion is sometimes referred to as sap, it is not in fact a sap, but rather an entirely different substance generated by the plant for protection. As resin moves along the trunk of a tree, it can trap a wide variety of organisms which become ensnared in the amber. When the resin hardens, it can preserve an entire organism, and sometimes even DNA is preserved.
Botanical material such as leaves, twigs, and bark is commonly found in amber, which is not terribly surprising, considering where it comes from. Amber fossils also typically include large numbers of microorganisms, which are ubiquitous in the natural environment, along with insects. In addition to insects, amber has also been known to contain frogs, lizards, and other small animals who simply happened to be in the wrong place at the wrong time.
Amber is not really an ideal fossil preservative. Amber fossils are biodegradable, and will break down when exposed to heat, ultraviolet light, and chemicals. This is one reason why people with amber jewelry need to take special care of it, to ensure that the amber is not damaged. Once an amber fossil has been uncovered and disturbed, steps need to be taken to preserve it, or it will eventually disappear. Even with proper care, the amber can eventually break down.
Numerous natural history museums maintain libraries of amber fossils for the purpose of study. These fossils can provide information about the history of life on Earth, with amber fossils comprising one piece of a complex puzzle. These fossils are also useful for learning about where various organisms lived, and what they ate, as sometimes it is possible to recover material from the guts of animals trapped in amber fossils. This material includes microorganisms found in the gut.