Classification of Rocks:
Building stones are obtained from rocks occurring in nature and classified in three ways.
  • Geological classification
  • Physical classification
  • Chemical classification
Geological Classification:
According to this classification, the rocks are of the following types.
  • Igneous rocks: Rocks that are formed by cooling of Magma (molten or pasty rocky material) are known as igneous rocks.  Eg: Granite, Basalt and Dolerite etc.
  • Sedimentary rocks: these rocks are formed by the deposition of production of weathering on the pre-existing rocks. Examples: gravel, sandstone, limestone, gypsum, lignite etc.
  • Metamorphic rocks. These rocks are formed by the change in character of the pre-existing rocks. Igneous as well as sedimentary rocks are changed in character when they are subject to great heat and pressure. Known as metamorphism. Examples: Quartzite, Schist, Slate, Marble and Gneisses.
Physical Classification:
This classification based on general structure of rocks. According to this, the rocks are classified into three types
  • Stratified Rocks: These rocks posses planes of stratification or cleavage and such rocks can be easily split along these planes Ex: sedimentary rocks
  • An stratified rocks: The structure may be crystalline granular or compact granular. Examples: Igneous rocks and Sedimentary rocks affected by movements of the earth.
  • Foliated Rocks: These rocks have a tendency to split up in a definite direction only. Ex: Metamorphic rocks.
Chemical Classification:
According to this classification rocks are classified into three types.
  • Siliceous rocks: In these rocks, silica is predominates. The rocks are hard; durable and not easily effected by weathering Agencies. Ex: Granite, Quartzite, etc.
  • Argillaceous Rocks: In these rocks, clay predominates. The rocks may be dense and compact or may be soft. Ex: slates, Laterites etc.
  • Calcareous rocks: In these rocks, calcium carbonate predominates. The durability to these rocks will depend upon the constituents present in surrounding atmosphere. Ex: Lime Stone, marble etc. 
Uses of stones:
  • Structure: Stones are used for foundations, walls, columns, lintels, arches, roofs, floors, damp proof course etc.
  • Face works. Stones are adopted to give massive appearance to the structure. Wall is of bricks and facing is done in stones of desired shades. This is known as composite masonry.
  • Paving stones: These are used to cover floor of building of various types such as residential, commercial, industrial etc. They are also adopted to form paving of roads, foot paths etc.
  • Basic material: Stones are disintegrated and converted to form a basic material for cement concrete, morum of roads, calcareous cements, artificial stones, hallow blocks etc.
  • Others: Stones are also used for (i) ballast for railways (ii) flux in blast furnace (iii) Blocks in the construction of bridges, piers, abutments, retaining walls, light houses, dams etc

Qualities of a good building stone:
The following are the qualities or requirements of a good building stone.
  • Crushing strength: For a good building stone, the crushing strength should be greater than l000kg per cm2.
  • Appearance: Good building stone should be a uniform colour, and free from clay holes, spots of other colour bands etc capable of preserving the colour for longtime.
  • Durability: A good building stone should be durable. The factors like heat and cold alternative wet and dry, dissolved gases in rain, high wind velocity etc affect the durability.
  • Fracture: For good building stone its fracture should be sharp, even and clear.
  • Hardness: The hardness greater than 17, treated as hard used in road works. It is between 14 to 17, medium hardness, less 14 said be poor hardness.
  • Percentage wear: For a good building stone, the percentage wear should be equal to or less than 3 percent.
  • Resistance to fire: A good building stone be fire proof. Sandstone, Argillaceous stone resists fire quite well
  • Specific gravity: For a good building stone the specific gravity should be greater than 8.7 or so.
  • Texture: A good building stone should have compact fine crystalline structure should be free from cavities, cracks or patches of stuff or loose material.
  • Water absorption: For a good building stone, the percentage absorption by weight after 24 hours should not exceed 0.60.
  • Seasoning: Stones should be well seasoned before putting into use. A period of about 6 to 12 months is considered to be sufficient for proper seasoning.
  • Toughness Index: Impact test, the value of toughness less than 13 – Not tough, between 13 and 19 – Moderate, greater than 19- high