Preserving a specimen via amber is a really good way to keep a lot of detail. It starts out as a resin that comes from coniferous trees (trees that produce their seeds in cones, i.e. pine trees), very much like a tree sap. As the resin surfaces on the branches or trunk, it hardens. This is a method the trees use for protection from predators such as insects or browsing of larger animals. It can also protect cuts or breaks that happen to the tree, acting as a shield from more outside harm while the tree repairs itself. The resin will engulf the insect (or whatever is destined to become a fossil) completely and then hardens around it, preserving whatever it is exact way it looks. For the resin to harden into the next stage, something called copal. This is harder and more durable than the hardened resin, but it can still be destroyed rather easily. It takes thousands of years (sometimes even millions) for the resin to become amber, and it needs the right set of conditions for it to preserve correctly. It needs a lack of oxygen, so it works best if it’s submerged in sediments under water or something like that. The oxygen will eat away at the resin. The terpenes (a large class of hydrocarbons often found in many coniferous trees) in the resin is what causes the substance to harden into amber, by chemically linking themselves, like they’re being fused together.
“The specimen...is a classy piece of Baltic Oligocene amber trapping the best preserved spider in existence.”
"Amber." Fossil Preservation . Web. 27 Jan 2011. <http://www.t-rat.com/Pages/FossilPreservation.html>.
This is a lizard preserved in amber, showing that not only small insects can be preserved this way.
"Amber With Ancient Lizard Caught Inside by Travelpod Member Kris." tripadvisor. Web. 10 Feb 2011. <http://tripwow.tripadvisor.com/slideshow-photo/amber-with-ancient-lizard-caught-inside-by-travelpod-member-kris-boca-chica-dominican-republic.html?sid=10173182&fid=tp-7>.