Visual rock identification is a fundamental skill in geotechnical and civil engineering. By closely observing a rock's physical characteristics, engineers can make an initial assessment of its properties and suitability for construction. This method of field classification is referred to as the geological or lithological classification of rocks.

When classifying rocks on site, five key characteristics are typically evaluated:

Texture

Structure

Composition

Colour

Grain size

Let’s explore how each of these properties helps in identifying and classifying rocks in the field.

1. Texture of Rocks

The texture refers to the size, shape, and arrangement of grains or crystals in the rock. Based on visual examination, textures can be grouped into the following categories:

a. Crystalline

Composed of clearly visible interlocking crystals

When scratched with a knife, no particles dislodge

Appears strong and tightly bonded

b. Indurated

Crystals are too fine to be seen with the naked eye

Rock feels hard and dense

No particles come loose when scratched with a knife

c. Crystalline–Indurated

Intermediate between crystalline and indurated

Grains are visible, but finer than crystalline rocks

Still resists disintegration when scratched

d. Compact

Very fine grains, tightly packed

Scratching with a knife may produce powder

Generally appears dense and solid

e. Cemented

Grains are visible, ranging from medium to coarse size

Particles can be easily detached, even with a fingernail

Indicates a weaker bond between grains

2. Structure of Rocks

Structure describes how minerals or grains are arranged and oriented within the rock body. There are several types:

a. Homogeneous

Grains are randomly oriented

Uniform appearance throughout

b. Lineated

Grains show a preferred alignment in one or more linear directions

Rock has a banded or striped look

c. Intact-Foliated

Mineral grains are aligned in a distinct planar pattern

Indicates foliation without significant fractures

d. Fracture-Foliated

Planar structures are intersected by fractures or cleavage planes

Common in metamorphic rocks like schists

3. Composition of Rocks

The mineral composition—especially the presence of calcite (calcium carbonate)—affects both mechanical strength and reactivity. Based on calcite content, rocks are classified as:

a. Non-Calcareous

Contains no calcium carbonate

Typically composed of silicates or other non-calcite minerals

b. Part Calcareous

Primarily non-calcareous but contains calcite as a binding material

Has a mixed composition

c. Calcareous

Composed mainly of calcite

Reacts with dilute hydrochloric acid

4. Colour of Rocks

Colour can offer clues about the chemical makeup of rocks:

a. Light-Coloured Rocks

Generally acidic in nature

Often rich in feldspar and quartz

Pure calcareous rocks are usually light

b. Dark-Coloured Rocks

Tend to be basic or mafic

Contain ferromagnesian minerals like pyroxene, amphibole, or biotite

Partially calcareous rocks often appear darker

5. Grain Size of Rocks

Even without knowing the rock’s origin, grain size alone can be used for classification:

a. Coarse-Grained

Grain size > 2 mm

Grains or particles are easily visible and distinguishable

b. Medium-Grained

Grain size between 0.1 mm and 2 mm

Texture is smooth but granular under close inspection

c. Fine-Grained

Grain size < 0.1 mm

Individual grains cannot be seen with the naked eye

Conclusion

Classifying rocks in the field using geological or lithological methods provides valuable insights into their properties and suitability for construction, excavation, and other engineering applications. Understanding texture, structure, composition, colour, and grain size enables engineers to make quick and effective decisions without the need for immediate laboratory testing.