Geology 101 |
 Natural Stone Basics
The following was published in 2004 by the Marble Institute of America.
"Natural Stone: Building Blocks Through Time"
There are three primary geological classifications for natural stones used in construction applications. These classifications are igneous, sedimentary and metamorphic and reflect how the rock was formed.
IGNEOUS
Igneous rocks were molten matter deep in the earth; they cooled and took shape as the mass was pushed to the surface. Igneous rocks include granite, gabbro, basalt and porphyry. The mineral composition is primarily feldspar and quartz plus other minerals such as mica and hornblende that contribute to the color of the stone.
The grain structure can range from coarse to fine and is generally consistent throughout the stone, although some can show veining. Broad ranges of colors are available including reds, browns, blacks, whites, beiges, yellows, pinks, greys and greens. Granites are hard, dense stones that can be highly polished or finished in a variety of ways. They are generally suitable for exterior and interior applications.
SEDIMENTARY
Sedimentary rocks include limestones, travertines, sandstones and soapstones. They were formed by the accumulation of materials settling in basins and sea beds and becoming compacted over millions of years.
Limestones are a common building stone used throughout the world in colors ranging from grey to buff and some pastel shades of pink and yellow. Limestones are composed primarily of calcium carbonate. Some limestones are very hard and dense; others are softer and more porous. Harder limestones may take a polish while the softer ones will not.
Travertines are a crystalline form of calcium carbonate that developed in layered deposits adjacent to cold and warm mineral springs. The characteristic holes were formed by the percolation of the mineral waters through the deposit over millions of years. Travertines vary widely in hardness, density and porosity. They are typically light to dark beige although some have white to dark brown veining. Some travertines can be polished to a high gloss finish.
Sandstones are composed primarily of quartz crystals bonded together with silicon, calcium carbonate or iron oxide. The color of sandstone is determined by the amount of iron oxide present and the colors can range from light to dark including browns, reds, buffs and yellows. The density, porosity and hardness of sandstones will also vary depending on the specific quarry deposit.
Soapstones range in color from grey green to grey brown and are composed of talc and chlorite. The talc gives soapstone its characteristic slippery feel and makes it a very soft, but dense, stone that is used for ornamental carving. Its heat-retention qualities also make it appropriate for cladding for heating units.
METAMORPHIC
Metamorphic rocks include marbles, slates, quartzites, schist and gneiss. These rocks were formed as another type and over millions of years metamorphosed into the building stones with these familiar names.
Marbles are composed primarily of dolomite and calcite with a wide variety of other minerals contributing to the color and texture. Marble began as limestones and the colors, textures and veining are produced through metamorphic activity. A rainbow of colors from around the world is available including blacks, reds, pinks, yellows, whites, greens, blues and beiges. Commercially, any stone (except granite) capable of taking on a polish is called marble. However, a true marble is completely metamorphosized.
Slate is formed by the metamorphosis of clay and shale and is composed of quartz, mica chlorite. It is uniform in color and generally available in dark to light green, black, grey or dark red. Unless the surface has been honed smooth, slate can be recognized by its distinct cleft pattern.
Quartzite is compact and dense stone composed of quartz, mica and feldspar. It is metamorphosed sandstone and some can have a texture similar to slates.
Schist is further metamorphosed slate with a granular texture similar to granite. Some schists are marketed as bluestone. Gneiss is further metamorphosed schist. Gneiss-type stones are composed primarily of quartz and feldspar and are often confused with granites.
Serpentines are green in color and show banding throughout the stone. They are similar to marbles in character, but are composed of hydrated magnesium silicate. Serpentines are dense, but can be soft and difficult to polish to a high gloss.
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