The concept would be crystallization. As magma cools inside or outside of the earth it crystallizes into an igneous rock. Intrusive (formed inside crust) igneous rocks are typically more course grained and contain many different noticeable minerals (for example, granite). Extrusive (formed outside) igneous rocks are typically more porous and are more fine grained then intrusive rocks (example, obsidian).
While it's true that Fossils hardly even contain DNA, they can still provide numerous clues in order to identify and verify an organism.
For example, in terms of Dinosaurs, their bones and fossils have provided enough evidence to provide an evolutionary link with modern birds.
Carbon dating is another way to find the age of a fossil which provides more information on it's surrounding habitat and the animals it interacted with.
Cells don't grow very big because they have to be able to diffuse through material easily. Also, when cells are small and plentiful they are easy to replace when damaged.
