This is a beautiful, polished pebble of Stromatolites (Greysonia sp.) from the Miraflores Formation of Bolivia. They are Vendian (Pre-Cambrian) in age or approximately 650 million years old.
Stromatolites are the layered trace fossils
of microbial life, primarily cyanobacteria. Some of them date back an astounding 3.4 billion years, making them the oldest record of life on planet Earth. Stromatolites and Microbialites were typically formed in shallow water by the growth of layer upon layer of cyanobacteria, a single-celled, photosynthesizing microbe. These layers often form very beautiful, and colorful banded structures in the rock.
These oxygen producing cyanobacteria were so simple they lacked DNA packaging nucleus, but were responsible for possibly the largest changes the earth has undergone. They were the only major source of atmospheric oxygen critical for the development of more complex life.