Fossil stromatolite. This polished slice through a rock shows layers of a fossilised stromatolite. Each layer began as a covering of blue-green algae (cyanobacteria), which captured grains of sediment and became mineralised. As more layers were added to the top over thousands of years, a stromatolite, or rocky mound, gradually formed. Fossilised stromatolites have been found within rocks as old as 3.5 billion years. This sample measures 300 millimetres across.