Based on their origin of formation stones are classified into three main groups—Igneous, sedimentary and metamorphic rocks.
Igneous Rocks:
These rocks are formed by cooling and solidifying of the rock masses from their molten magmatic condition of the material of the earth.
Generally, igneous rocks are strong and durable. Granite, trap and basalt are the rocks belonging to this category;
Granites are formed by slow cooling of the lava under thick cover on the top. Hence they have crystalline surface.
The cooling of lava at the top surface of earth results in a non-crystalline and glassy texture. Trap and basalt belong to this category.
Sedimentary Rocks:
Due to weathering action of water, wind and frost existing rocks disintegrates.
The disintegrated material is carried by wind and water; the water being most powerful medium.
Flowing water deposits its suspended materials at some points of obstacles to its flow.
These deposited layers of materials get consolidated under pressure and by heat. Chemical agents also contribute to the cementing of the deposits.
The rocks thus formed are more uniform, fine grained and compact in their nature.
They represent a bedded or stratified structure in general. Sandstones, limestones, mudstones etc. belong to this class of rock.
Metamorphic Rocks: Previously formed igneous and sedimentary rocks undergo changes due to the metamorphic action of pressure and internal heat.
For example due to metamorphic action granite becomes greisses, trap and basalt change to schist and laterite, limestone changes to marble, the sandstone becomes quartzite and mudstone becomes slate.
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