Natural stones used in building.

Monday, February 7, 2011

The natural stones used in building may be classified by reference to their origin as:

(1) igneous
(2) sedimentary
(3) metamorphic.

Igneous stones
Igneous stones were formed by the cooling of molten magma as the earth’s crust cooled, shrank and folded to form heds of igneous rock. Of the igneous stones that can be used for building such as granite, basalt, diorite and serpentine, granite is most used for walls of buildings.

Granite consists principally of crystais of felspar, which is made up of lime and soda with other minerais in varying proportions and small grains of quartz and mica which give a sparkle to the surface of the stone. The granite that is native to these islands that is most used for walling is sometimes loosely described as Aberdeen granite as it is mined from deep beds of igneous rock near that town in Scotland. The best known Aberdeen granites are Rubislaw which is blue grey, Kemnay which is grey and Peterhead which is pink in colouring. Ah of these granites are fine grained, hard and durable and can be finished to a smooth polished surface. 

Aberdeen granites have been much used for their strength and durability as a walling material for large buildings and are now used as a facing material.

Cornish and Devon granites are coarse grained, light grey in colour with pronounced grains of white and black crystais visible. The stone is very hard and practically indestructible. Because these granites are coarse grained and hard they are laborious to cut and shape and cannot easily be finished with a fine smooth face.

These granites have been principally used in engineering works for bridges, lighthouses and docks and also as a walling material for buildings in the counties of their origin.

Sedimentary stone
Sedimentary stone was formed gradually over thousands of years from the disintegration of older rocks which were broken down by weathering and erosion or from accumulations of organic origin, the resulting fine particles being deposited in water in which they settled in layers, or being spread by wind in layers that eventually consolidated and hardened to form layers of sedimentary rocks and clays. Because sedimentary stone is formed in layers it is said to be stratified. The strata or layers make this type of stone easier to split and cut than hard, igneous stones that are not stratified. The strata also affect the way in which the stone is used, if it is to be durable, as the divisions between the layers or strata are, in effect, planes of weakness. A general subdivision of sedimentary stones is


The limestones used for walling consist mainly of grains of shell or sand surrounded by calcium carbonate, which are cemented together with calcium carbonate. The limestones most used for walling are quarried from beds of stone in the south-west of England, those most used being Portland and Bath stone. Because limestone is a stratified rock, due to the deposit of layers, it must be laid on its natural bed in walls. 

Metamorphic stones.
Metamorphic stones were formed from older stones that were changed by pressure or heat or both. The metamorphic stones used in building are siate and marbie.

Slate was formed by immense pressure on beds of clay that were compressed to hard, stratified siate which is used for roofing and as chis and copings in building. Riven, split, Welsh slate has for centuries been one of the traditional roofing materials used in this country. The stone can be split to comparatively thin siates that are hard and very durable.

The description marbie is used to include many stones that are not true metamorphic rocks, such as limestones, that can take a fine polish. In the British Isles true marbie is only found in Ireland and Scotland. Marbie is principally used as an internal facing material in this country.

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