Leaves and Invertebrates
Leaf fossils and invertebrates are often not removed fully from the rock, as they are too fragile. Instead, small air scribes are used to carefully remove rock from around the fossil. This reveals intricate structures in the fossils: veination in leaves, stripes on insects, spines on trilobites.
Sometimes, acids and other chemicals are used to prepare the fossils as well. This technique has to be very delicate, however, so as not to errode the fossils along with the rocks.
Fossil cystoids, before and after preparation
Most vertebrates are prepared out of the rocks entirely, though not all. Fish fossils, particularly, are often left in the matrix, as was the famous Archaeopteryx specimen. Small air scribes, dental picks, water, acetone glues, and a lot of patience are generally the tools of choice. Preparators have to work slowly, so as not to damage the bone. In fact, it takes about 300-500% more time to prepare a fossil than it does to take it out of the field in the first place. Every bit of exposed bone has to be carefully reinforced with superglues, as well, so it doesn't crumble away. It's a tedious process, but a necessary one. Without preparation, not one fossil could be studied or put on display in a museum.
A saber-toothed cat skull, before and after preparation
For more information, visit the DMNS Follow a Fossil site, and read the preparation section.
Image credit: Marc Behrendt Preparation