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Rocks come in just about any size, shape and colour. Sand, gravel and clay are all types of rock.
Rocks are often classified according to how they formed in the earth’s crust. There are three groups of rocks : igneous, sedimentary and metamorphic.

Igneous rocks are formed when lava or molten rock cools and hardens. The word 'igneous' comes from a word meaning 'fire'. Basalt is an igneous rock 'born' from a volcano. Granite is another type of igneous rock but it is formed inside the earth rather than from a volcano.

Sedimentary rocks are made from other rocks. They form over millions of years from rock particles (or organic remains) that have settled in layers, often at the bottom of ancient seas. Sandstone (from sand), limestone (from the skeletons of billions of microscopic sea animals) and conglomerate (made from rocks and pebbles of different sizes) are typical examples. Sometimes you can see the layers in sedimentary rocks and find fossils in them.

Metamorphic rocks are the third type of rock. They are formed when intense heat or tremendous pressure changes existing rocks.  The existing rocks are 'cooked' and change their form and appearance. The word 'metamorphic' means to 'change form'. Quartzite is a metamorphic rock formed when sandstone is 'cooked'. Similarly, the rock called marble is formed from limestone.