Rocks can be classified on the basis of their physical appearance. Through visual inspection we can ascertain the engineering properties of rocks. Field classification of rocks gives us logical insights about the suitability of rocks for engineering purpose. This type of classification is known as geological or lithological classification of rock.

While observing rocks we should describe the following properties of rocks, such as:

  • Texture
  • structure
  • Combination
  • Colour
  • grain size

Let us know how to describe the above properties of rock.

field classification of rocks

geological or lithological classification of rock

1. Texture

Rock material may belong to any of the following texture groups.


  • These types of rocks are composed of visible interlocking crystals or crystal grains.
  • Scratching with a knife does not remove particles from the mass of the rock.


  • Interlocking crystals or crystal grains are not visible to the naked eye.
  • Rocks contain fine grained material
  • Particles are not removed from the rocky mass when scratched with a knife.

crystalline – induced

  • Individual crystal grains or crystal aggregates are finer than the crystalline structure but coarser than the solid structure.
  • The rash does not come out when scratched with a knife


  • The particle or grain is tightly packed
  • grains are fine
  • Scratching with a knife removes particles or powder from a mass of rock


  • grains visible to the naked eye
  • Grain size ranges from medium to coarse-grained structure
  • Particles come out from the mass of the rock even when scratched with a fingernail.

2. structure

The composition of the rock mass refers to the orientation of the mineral grains within the rock mass. The different types of structures are as follows.


If the grains and crystals have random orientation, the structure would be said to be homogeneous.

outlined in low cost

If the mineral particles have a preferred orientation in a particular linear direction/direction, the structure will be known as linear.


When the minerals in a rock mass have a planar nature’s preferred orientation


When a planar structure is closing or having an initial fracture such as a bed plane or cleavage plane

3. Combination

The mechanical and physical characteristics of rock are greatly influenced by the presence of calcite. Therefore on the basis of a presence of calcite the rock can be classified as follows.

non calcium

Acalcareous rock materials are those in which calcium carbonate is absent.

part calcareous

The rock consists mainly of non-rock material. The gram content exists as a bond between the grains.


rock material composed mainly of calcite

4. Colour

light color

  • rocks are acidic in nature
  • Rocks are generally feldspathic
  • Pure gram rocks are light in color

dark color

  • Rocks are not acidic in nature
  • Rocks usually contain the mineral ferromagnesium.
  • The rocks of the gram are dark in color.

5. grain size

Sometimes rocks are classified on the basis of their grain size. In such cases the origin or type of rock is not so important. On the basis of grain size, rock materials are classified into 3 groups, as given below.

coarse grained

When the size of the particles or grains is 2 mm. be greater than

medium grained

When the particle or grain size varies from 2 mm to 0.1 mm. be between

fine grained

When the particles are less than 0.1 mm in size and the individual grains are invisible to the naked eye

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.