This rock, which testifies to the first signs of life on Earth, is found at the beginning of ‘Biography of the Earth’, in the Archaean period (from 3,800 to 2,500 million years ago). It is is a fragment of a sedimentary formation of the type of banded iron (Banded Iron Formation (BIF))
If you look closely at the polished surface of this sample, you will see some red and other black bands that correspond to the hematite and magnetite minerals, respectively. These minerals are iron oxides that are formed in the presence of oxygen and their origin was probably linked to the activity of cyanobacteria. These micro-organisms played a fundamental role in the origin of life on Earth, as their activity on the seabed generated a very high increase in oxygen and created an oxidising atmosphere.