Rocks Tell the Story of the Earth. The Earth is made of rock, from the tallest mountains to the floor of the deepest ocean. Thousands of different types of rocks and minerals have been found on Earth. Most rocks at the Earth's surface are formed from only eight elements (oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and sodium), but these elements are combined in a number of ways to make rocks that are very different.

Rocks are continually changing. Wind and water wear them down and carry bits of rock away; the tiny particles accumulate in a lake or ocean and harden into rock again. The oldest rock that has ever been found is more than 3.9 billion years old. The Earth itself is at least 4.5 billion years old, but rocks from the beginning of Earth's history have changed so much from their original form that they have become new kinds of rock. By studying how rocks form and change, scientists have built a solid understanding of the Earth we live on and its long history.

Flint is my favourite rock. Where I live in Norfolk, England, it is used widely in the building industry. It is a massive compact variety of quartz, made from fine grained silica. Normally grey to brown or nearly black in colour, it breaks with a conchoidal fracture and sharp edge, and as it is incredibly hard, you can see why it was so popular with cavemen!

It is believed to be one of the first methods by which neanderthal man made fire - hit two flints together and you get an orange spark, and it gives off a sulphurous odour..