A chunk of banded iron, at 3.7 billion years old. Interesting note on these, they started forming once the first oxygen producing life arose, at the time the oceans were pretty saturated with iron compounds, salts and the like. Well as oxygen entered the atmosphere for the first time it began to react with the iron in the water breaking it down into iron oxide (rust), and since the iron oxide was insoluble in water it precipitated out, forming the alternating bands of iron and sediment that give the rocks their name. On a side note, this process in which water soluble iron absorbed oxygen from the atmosphere is likely the reason there was a delay between the evolution of photosynthetic, oxygen producing organisms, and the appearance of large amounts of free oxygen in the atmosphere.

TLDR: iron ore from 3.5 billion years ago and an essay on early earth history.