Stone and cast stone

The term stone refers to natural rocks after their removal from the earth’s crust. The significance of stone as a building material is illustrated by widespread prehistoric evidence and its sophisticated use in the early civilisations of the world, including the Egyptians, the Incas of Peru and the Mayans of Central America. Geologically, all rocks can be classified into one of three groups: igneous, metamorphic and sedimentary, according to the natural processes by which they were produced within or on the earth’s surface.

Igneous rocks are the oldest, having been formed by the solidification of the molten core of the earth or magma. They form about 95% of the earth’s crust, which is up to 16 km thick. Depending on whether solidification occurred slowly within the earth’s crust or rapidly at the surface, the igneous rocks are defined as plutonic or volcanic, respectively. In the plutonic rocks, slow cooling from the molten state allowed large crystals to grow, which are characteristic of the granites. Volcanic rocks such as pumice and basalt are fine-grained and individual crystals cannot be distinguished by eye; thus, the stones are visually less interesting. Dolerites, formed by an intermediate rate of cooling, exhibit a medium-grained structure.

Basalt is a fine-grained stone nearly as hard as granite. It can be melted at 2400 °C and cast into tile units which are deep steel grey in colour. Annealing in a furnace produces a hard virtually maintenance-free, shiny textured surface flecked with shades of green, red and bronze.

Although basalt and dolerite have not been used widely as building stones they are frequently used as aggregates, and cast basalt is now being used as a reconstituted stone.

Sedimentary rocks are produced by the weathering and erosion of older rocks. The natural bedding planes associated with the formation of the deposits may be thick or thin but are potentially weak; this is used to advantage in the quarrying process. In masonry, to obtain maximum strength and durability, stones should be laid to their natural bed except for cornices, sills and string courses which should be edge-bedded.

Deposits of sand cemented together by calcium carbonate, silica, iron oxide and dolomite produce calcareous, siliceous, ferruginous and dolomitic sandstones, respectively. Depending on the nature of the original sand deposit, the sandstones may be fine or coarse in texture. Sandstones range in colour from white, buff and grey through to brown and shades of red depending on the natural cement; they are generally frost-resistant.

Limestones consist mainly of calcium carbonate, either crystallised from solution as calcite or formed from accumulations of fossilised shells deposited by various sea organisms.They are generally classified according to their mode of formation. Many colours are available ranging from off-white, buff, cream, grey and blue.

Metamorphic rocks are formed by the recrystallisation of older rocks, when subjected to intense heat or pressure or both, within the earth’s crust. Clay is metamorphosed to slate, limestone to marble and sandstone to quartzite.

Slate is derived from fine-grained sand-free clay sediments. Slate can be split into thin sections (typically 4–10 mm for roofing slates) giving a natural riven finish, or it may be sawn, sanded, fine rubbed, honed, polished, flame textured or bush hammered.

Marble is metamorphosed limestone in which the calcium carbonate has been recrystallised into a mosaic of approximately equal-sized calcite crystals. The process, if complete, will remove all traces of fossils, the size of the crystals being largely dependent on the duration of the process.

Quartzite is metamorphosed sandstone. The grains of quartz are recrystallised into a matrix of quartz, producing a durable and very hard-wearing stone used mainly as a flooring material. The presence of mica allows the material to be split along smooth cleavage planes, producing a riven finish. Quartzite is available in white, grey, grey-green, blue-grey and ochre colours.

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