Building stone

Building stone is any naturally occurring stone that is used for building construction. The three rock types (igneous, metamorphic, and sedimentary) are all utilized in building stone. The physical characteristics of each type of rock, such as hardness, color, and texture, determine how the stone is used.

Various Forms

Building stone is any kind of rock that has supplied humankind with the material to erect monuments and edifices throughout history. The pyramids of Egypt, the temples of Greece, and the skyscrapers of the modern world have all utilized various forms of building stone. The ancients used blocks of stone stacked like building blocks, whereas the modern builder uses a thin veneer of stone anchored to the exterior of a building frame. Special physical properties of the various stone materials lend themselves to these various usages. Because of these special physical properties, only a small percentage of the earth’s rock material can be classified and utilized as building stone. Granite, an igneous rock that forms the core of the continents, is a good example. Granite is abundant, but because of mineralogical variations or structural weakness, only a small percentage of the granite areas will yield quarry blocks of suitable dimensions for building stone.

Building stone is quarried, or excavated, in most countries and then may be shipped around the world. Quite commonly, granite blocks from Brazil are shipped to Canada for sawing and finishing and then shipped to a construction site in a city in the United States. Similarly, building stone from the United States has been shipped to Italy for sawing and finishing and then shipped to England for installation on buildings. The average quarry block weighs 15 to 20 tons. These quarry blocks are sawed into slabs that vary in thickness from half of an inch to four inches. Whether the building stone is limestone, marble, granite, slate, or sandstone, the quarry block must be solid enough to yield slabs of competent rock (that is, rock capable of withstanding stress). These slabs are further cut on diamond saws to the desired shape and dimensions and then finished to the requirements of the architect or owner.

Design Criteria

Buildings constructed before or around the turn of the twentieth century were built with rectangular pieces 6 inches thick or greater. Rectangular stone forms both the supporting structure of the walls and the exterior protection of the building. Most buildings built since the 1950s, however, require that the building stone be cut into thin panels less than 3 inches thick and fastened to the exterior of the steel supporting structures. This use of stone demands that more engineering and design criteria be used to establish acceptable versus unacceptable building stone. The processes by which the architect, engineer, and geologist select and qualify a particular stone for usage help to determine the aesthetic and physical characteristics of the stone. The architect usually establishes the design criteria for the size and shape of the stone, looking at the stone for such aesthetic aspects as color and texture. The engineer and geologist then determine the suitability of the stone within the design plan.

The geologists and engineers involved with quarrying and fabricating building stone look for deposits that are uniform in texture, color, and structural integrity. The exploration of new areas requires researching and reviewing geological maps, aerial photographs, and geological libraries. Ground sleuthing or field mapping and reconnaissance are the next steps in delineating a potential quarry location. Rock sample collection, mineralogic identification, and the testing of physical properties are conducted to determine whether the stone would be suitable for the specific use. The final use of the building stone varies from a stone block size of 100 cubic feet (about 4.6 feet on a side, or 4 by 5 by 5 feet) to a rubble stone size of 1 cubic foot. Some blocks smaller than 100 cubic feet can be used to produce tiles and novelties, but for the most part the major emphasis is on quarrying larger blocks of stone.

Quarrying large blocks of stone requires geological uniformity of the rock type over an ample enough area to justify opening a quarry. This area needs to be of an adequate size to open a quarry that will yield uniform material for a time span of ten to twenty years.

Color and Texture

The color of the stone, so critical to the architect’s design criteria, is studied by polished hand samples or test block extraction from the potential quarry area. The mineralogical content of the various granites will determine the color consistency and stability. Petrologic microscope work and thin-section studies are an important part of the search for suitable granite. Color stability is as important a factor as color uniformity; there are numerous dark limestones that, while unsuitable for exterior use, are beautiful in an interior lobby. Some minerals can make stone unsuitable. Even tiny amounts of pyrite, for example, will weather to make unsightly rust stains on the surface, releasing a sulfuric acid that attacks the stone.

Texture and mineral fabric are also important considerations with regards to the suitability of a particular stone. The use of the stone will depend on whether it has a fine or coarse texture. Interlocking mineral grains, a well-cemented matrix, and nonsoluble minerals are also important characteristics. Many fine-textured marbles are “sugary” and soft because of poorly interlocked calcite grains, whereas a similar fine-grained marble, with an interlocked mineral fabric and low porosity to surface water, may be tough and resistant to atmospheric corrosion.

Soundness and Strength

The major criterion in determining the suitability of a building stone is the soundness of the deposit. The soundness is a measure of the natural fractures and the strength of the stone. Geological conditions during the formation of various rock types will determine soundness. In a sedimentary rock, the bedding (foundation or stratification) and jointing (fracturing without displacement) will determine the block sizes capable of being produced. In an igneous and metamorphic rock, the schistosity (tendency to split along parallel planes), cleavage (tendency to split along closely spaced planes), and jointing will be contributing factors in determining the degree of soundness. In an igneous and metamorphic rock mass, the internal stresses need to be measured and understood before determining potential block sizes. In the years prior to diamond-cutting tools and hydraulic handling equipment, cleavage, jointing, and bedding were used to help pry out blocks of stone. With the development of such tools, quarry workers can cut out more rock masses for building stone.

Rock strength is another important criterion of a building stone. Test engineers use specific laboratory equipment to determine the flexural and compressive strength of each building stone. Depending on the planned usage, water absorption and abrasive wear test data are also important for knowing how a particular rock will react under certain applications. The test program may include producing test specimens of the finished product and installing them in mock-up panels.

The Building Stone Industry

Building stones have been used to produce comfortable habitations ever since cave dwellers blocked up cave entrances for protection and warmth. In the modern world, architects utilize building stone to clad the exteriors of modern buildings as a protective shield for structural steel supports. In the entrances of these same buildings, designers take polished building stone and clad interior lobbies for durability and beauty. Slate, marble, granite, limestone, and sandstone are all utilized as building stone. These materials occur throughout the world in easily exploitable deposits. From the mid-1800s to the mid-1900s, the building-stone industry in North America was both the impetus and nucleus for towns, railroads, and machine tool industries across the continent. From the 1900s through the 1930s, many technological advances in quarry drilling and finishing equipment were fostered by the building-stone industry. The 1950s through the 1970s saw many adaptations to the demands of architects and designers. Innovative technology was developed to gain competitive advantage over other producers and to satisfy the need to exploit deeper and more solid deposits of stone. This technology ranged from manual labor in the mid-1800s to the use of steam, electricity, and compressed air by the early 1900s. The equipment and tools developed in the early 1900s remained relatively unchanged until the 1950s, at which time the more sophisticated use of carbides and diamonds for cutting and a high grade of steel for drilling allowed for the more efficient excavation of stone. The use of hydraulics and electronics has also allowed for a higher degree of automation and cost savings.

In North America, the building-stone industry has shrunk from more than a thousand active quarries in the 1920s—with more than a dozen locations where quarries, mills, and shops were an integral part of the local economy—to a number in the hundreds. During the 1950s and 1960’, the building-stone industry in the United States nearly became extinct before stone became fashionable again. Yet the trend toward the increased use of stone in the late 1980s has suggested that the building-stone industry will become an important factor in local economies once again.

Principal Terms

granite: an igneous rock that is known for its hardness and durability; in modern times, it has been used on the exterior of buildings, as it is able to resist the corrosive atmospheres of urban areas

igneous: rock that was formed from molten material originating near the base of the earth’s crust

limestone: a sedimentary rock that can be easily shaped and carved; currently, it has gained acceptance as a thin veneer

marble: a metamorphic rock that has been used since Grecian times as a preferred building stone; it is known for its ability to be carved, sculptured, and polished

metamorphic: rock that was formed by heat and pressure; tectonic, or mountain-building, forces of the earth’s crust create and alter the mineral composition and texture of the original rock material

sandstone: a sedimentary rock that is known for its durability in resisting abrasive wear; it is likely to be used for paving stone

sedimentary: most commonly, rock that was formed by marine sediments in an ocean basin; it usually shows depositional features and may include fossils

slate: a metamorphic rock that has a unique ability to be split into thin sheets; some slates are resistant to weathering and are thus good for exterior use

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