Rocks are composed of minerals. Some rocks are made of only one type of mineral, for example, marble (calcite), while others are composed of a mixture of different minerals.

In the simplest system, rocks can be divided into three main groups called igneous, sedimentary and metamorphic.

(i) Igneous rock is formed from molten rock that has cooled and solidified: the rate of cooling determines the crystal size. Often being hard, igneous rock is typically used where hard-wearing surfaces are needed, e.g., kitchen table tops and paving. Granite, Dolerite, Basalt are some e.g of Igneous rock

(ii) Sedimentary rocks Rock that is exposed on the Earth’s surface is often the target of chemical attacks in the atmosphere. This process is called weathering, and leads to the breakdown of the rock. The products of weathering or mechanical breakdown are carried away by water, wind or ice by a process called erosion. These sediments are converted to sedimentary rock in a process called lithification. This means that the particles of the sediment are ‘cemented’ together.

Sedimentary rocks are classified according to the type of sediment formed:

Clastic sediments - These are sediments formed from rock particles and fragments (clay minerals, sand grains and rock fragments). Examples of this type of rock . breccia, sandstone, shale and mudstone.

Chemical sediments - These are sediments formed from the material from a liquid solution settling down. Examples of this type of rock are limestone, dolomite and chalk.

(iii) Metamorphic rocks are always formed through the recycling of other rocks. High temperatures and pressures deep in the Earth provide the conditions to modify rock by baking and squeezing.  Metamorphic rocks are normally very hard and often aesthetically appealing. For these reasons they are typically used for decorative purposes, such as statues or the interiors of important buildings. Examples of metamorphic rock are: slate, schist, gneiss, marble, and quartzite.