Last reviewed 16 Sept 2026 · 9 min read
Classification of rocks
Geological classification
| Type | Formation | Examples |
|---|---|---|
| Igneous rocks | Cooling and solidification of molten magma — intrusive (plutonic), coarse-grained (cooled slowly deep inside) or extrusive (volcanic), fine-grained (cooled quickly at the surface) | Granite (plutonic), basalt and trap (volcanic), dolerite (hypabyssal), syenite, diorite |
| Sedimentary rocks | Deposition and consolidation of sediments by water, wind or ice — usually stratified | Sandstone, limestone, shale, laterite, conglomerate, gypsum |
| Metamorphic rocks | Alteration of igneous or sedimentary rocks by heat and pressure | Marble (from limestone), quartzite (from sandstone), slate (from shale), gneiss (from granite), schist |
Physical classification
- Stratified rocks — in layers (sedimentary rocks such as sandstone, limestone); can be split along bedding planes.
- Unstratified rocks — no layers; compact crystalline structure (igneous rocks such as granite, trap).
- Foliated rocks — tend to split along a definite direction (metamorphic rocks such as slate, gneiss, schist).
Chemical classification
| Class | Main constituent | Examples | Remarks |
|---|---|---|---|
| Siliceous | Silica (SiO₂) | Granite, quartzite, sandstone (silica-cemented) | Hard, durable, resist weathering |
| Argillaceous | Clay (alumina) | Slate, laterite, shale | Dense and compact or soft and brittle depending on type |
| Calcareous | Calcium carbonate | Limestone, marble | Affected by acids and polluted (industrial) atmospheres |
Common building stones and uses
| Stone | Characteristics | Uses |
|---|---|---|
| Granite | Igneous, hard, strong, durable, takes a good polish; high crushing strength; poor fire resistance (cracks on heating) | Bridge piers, docks, retaining walls, steps, kerbs, facing, flooring, road aggregate |
| Basalt and trap | Igneous, hard, tough, dark, difficult to dress | Road metal, rubble masonry, railway ballast, aggregates |
| Sandstone | Sedimentary, stratified, porous; strength depends on cementing material (siliceous best) | Walling, ashlar work, flooring, ornamental work |
| Limestone | Sedimentary, calcareous; varieties include compact and oolitic | Flooring, walling, manufacture of lime and cement, road metal (hard varieties) |
| Marble | Metamorphic, crystalline, takes a high polish, variety of colours | Flooring, facing, decorative and ornamental work, statues |
| Slate | Metamorphic, splits into thin non-absorbent sheets | Roofing, damp-proof courses, sills, cisterns |
| Quartzite | Metamorphic, very hard, durable, difficult to work | Rubble masonry, road metal, concrete aggregate |
| Gneiss | Metamorphic, banded, strong | Street paving, rough stone work |
| Laterite | Residual (weathered) rock rich in iron and aluminium oxides; soft when quarried, hardens on exposure | Building blocks in coastal areas (e.g. Kerala, Konkan), road metal |
| Kankar | Impure limestone nodules | Lime manufacture, road metal for low-traffic roads |
Qualities of a good building stone
- Crushing strength — adequate for loads; good structural stones commonly have high strength (e.g. granite and trap rank highest; many sandstones and limestones are lower).
- Appearance — uniform colour and texture for face work; free from clay holes, spots and cracks.
- Durability — resistance to weathering (rain, frost, wind, temperature changes, chemicals).
- Hardness — resistance to abrasion (important for floors, pavements, steps); measured by Mohs' scale.
- Toughness — resistance to impact (road metal).
- Low porosity and water absorption — porous stones absorb water and deteriorate by frost and chemical action (water absorption for good stones is low, commonly below about 5%).
- Specific gravity — heavier stones (specific gravity above about 2.7) are generally more compact and durable, suited to dams and retaining walls.
- Fire resistance — stones with minerals of different expansion (e.g. granite with quartz) crack in fire; argillaceous stones resist fire better.
- Workability and dressing — ease of cutting and dressing (economy).
- Texture — fine, compact, crystalline structure is desirable.
- Seasoning — freshly quarried stones contain quarry sap and should be seasoned (allowed to dry) before use, commonly for several months.
- Availability and cost.
Tests on stones
| Test | Purpose / procedure |
|---|---|
| Crushing strength test | Cube specimens tested in a compression testing machine; strength = failure load ÷ area |
| Water absorption test | Dry specimen immersed in water (24 hours or boiling); absorption |
| Specific gravity | ( dry weight, weight in water) — heavier stones more compact |
| Hardness test (Mohs' scale) | Scratching with minerals of known hardness; also Dorry's abrasion test |
| Impact (toughness) test | Resistance to repeated blows of a hammer (as in aggregate impact tests) |
| Attrition (Deval) test | Rate of wear of stone pieces rotated in a cylinder — for road stones |
| Acid test | Stone immersed in dilute hydrochloric (or sulphuric) acid — sharp edges and firm grains indicate resistance to acidic atmospheres; calcareous stones effervesce |
| Crystalline test | Microscopic examination of crystals |
| Freezing and thawing (frost) test | Repeated cycles in a freezing mixture — weathering resistance; Brard's test uses sodium sulphate crystallisation |
| Smith's test | Stone chips shaken in water — clear water indicates absence of soluble or earthy matter |
Mohs' scale of hardness
| Hardness | Mineral | Hardness | Mineral |
|---|---|---|---|
| 1 | Talc | 6 | Orthoclase (feldspar) |
| 2 | Gypsum | 7 | Quartz |
| 3 | Calcite | 8 | Topaz |
| 4 | Fluorite | 9 | Corundum |
| 5 | Apatite | 10 | Diamond |