Natural Stone

Natural cut stone as a building material is derived from quarried rock; divided into three types, the classifications relate to how the rock was formed geologically. Igneous rock, such as granite, is formed by the crystallization and cooling of molten magma. It forms the bases of continents and mountains ranges, and is relatively hard and homogenous.

Sedimentary rock is created by a more complicated progression when weather and water movement erodes igneous rocks, forming gravel, sand and silt. These are deposited on the bottoms of oceans, lakes and river beds, along with minerals from sea water and marine organisms.

Particles are bonded together in a process known as lithification. Rock that is sedimentary is what covers the surface of most continents, including soils, clays and other fine strata. Limestone and sandstone are the two most common sedimentary stone materials.

Metamorphic rock is formed by extreme pressures and heat inside the earth metamorphing igneous and sedimentary rock into one crystalline mass. These are hard, dense rocks with the same composition chemically as the rock from which they came. Examples of metamorphic rock are marble, ref-formed from limestone, gneiss, from granite, and slate, from shale.

Making Natural Stone

In earlier days of masonry construction, rocks were gathered from nearby fields, carried to the site of construction and used as they were, with no dressing. Any projections that did not fit the intended use were chiseled or knocked off with hammers.

Sedimentary rocks taken from river banks or shorelines was also easy to use, it readily cracked into usable, rectangular shapes. As the art of masonry developed, quarrying and dressing came into use, the amount of finishing varying depending on the intended use of the stone.

Rock is quarried from within the ground as well as from the exposed side of cliffs, canyons and rock outcroppings. Rock found nearer to the surface tends to be weathered with small fissures and is mostly used for rubble walls and foundations. Manual quarrying continued well into the 19th century. Rocks were split off in large blocks by drilling holes in layers of sedimentary rock, then driving in iron or wooden wedges with sledge hammers.

Wooden wedges were soaked in water to cause their expansion and crack the stone block. Modern methods include diamond high-speed power drills and saws, pneumatic shears and air hammers. Following quarrying, large stone blocks are broken or cut with saws to their final size for ease of shipping and handling.

Dressing Stone

Although in a rubble wall, roughly rectangular stone can be used straight from the quarry, for more precise masonry, stone must be squared up with chisel and mallet or pick axes. Exposed faces may be finished flat or roughened to appear more natural. Following dressing, natural stone is either polished to a shine, chiseled with surface pattern tooling, or left with its distinctive natural look.

Properties of Stone

Wide variations exist in the appearance and usefulness of stone. Between kinds of stone and also from one sample of stone to another, there are variations in hardness, color, texture, weathering resistance and strength.

In terms of strength, igneous and metamorphic rock types are stronger than sedimentary rocks. In a stone building, architecture makes use of the fact that stone can withstand much compression force loads, but is relatively weak in tension and torsion strength.

This is why you see more buildings with arched window openings, where the stone blocks are in compression all around, than buildings with long stone lintels, where the block would be in torsion and tension and prone to breakage in the middle. In modern construction, natural stone is not used as structural load bearing members, but mostly as external facing and flooring, so strength is not of as much concern.

Moisture and Pollution Resistance

Moisture resistance is a desirable property in masonry stone, because moisture related problems are the cause of the majority of deterioration in masonry. Resistance to moisture is determined by the stone’s breathability, which in turn is determined by its permeability and porosity.

Porosity as rule is higher in sedimentary rocks, while igneous and metamorphic rock is low in porosity, as it has a lower amount of pores per volume. Permeability is affected by the size and amount of voids within the stone, with narrower voids transporting liquids the most efficiently.

Resistance of a stone to acids is a gauge of how good or poorly it will survive pollutants like acid rain. Especially vulnerable is stone composed mostly of carbonate minerals, like marble, limestone and calcareous sandstones. Siliceous sandstones and other silica based stones are generally fairly resistant to pollution and acids. Stone with low acid resistance that has been polished will rapidly lose its polish in environments with acid rain.

Marble is particularly prone to deterioration from pollutants, and most types are not well suited to use on building exteriors. Marble, which is more easily worked with than other masonry stone, is used mainly for decorative applications such as inlays and statuary. Indeed, the rapid decay of many of the historic monuments from the ancient Greek and Roman eras seen today are due to growing atmospheric pollutant levels.

Granite is dense, hard and has low porosity and permeability. This makes most granite types durable and wear resistant. Traditionally it was used for areas contacting the earth such as foundations and exterior steps, usually in buildings of sandstone or limestone.

Limestone has a good balance of workability with resistance to weathering, which accounts for it’s widespread use in construction. Composed mostly of calcium carbonate and dolomite, limestone is a sedimentary rock created from fragments of marine organisms like coral, bone and shell.