Common Descriptions & Uses
of Natural Stone
Common Stone Types
Scientific versus Commercial Definition
Stones,
and the minerals of which they are composed, have been studied with keen
interest in the earth science fields for centuries. Geology is the
study of the formation and history of the earth, while petrography
is the study of rocks and the minerals of which they're made. Geologists and
petrographers worldwide have defined hundreds of different rock types, based on
their mineral composition, texture, and method of formation. Commercially, the
use of the exact scientific rock definition would be a cumbersome and
unnecessarily complicated practice. Furthermore, there are many rocks which are
not clearly within one definition or another, but rather "straddling the
fence" between two definitions. This point is further elaborated by the
U.S. Geological Survey (USGS), Department of the Interior:
Scientific and commercial descriptions of various
dimension stone types overlap. The scientific description of dimension stone
types is focused primarily on the stone's geographic locality and mineralogical
composition, whereas the commercial description is focused primarily on the
locality and color of the stone.
Historically, it has been commercial practice to group
stones within performance and behavioral groups as opposed to true scientific
definition. This is recognized in several in ASTM International standards.
While scientifically there are hundreds of rock type identifications, only nine
groups are commonly acknowledged commercially: Granite,
Limestone, Marble, Onyx, Quartzite, Sandstone, Serpentine, Slate, Soapstone,
and Travertine. This means that some rocks are included in groups
which are not perfectly coincident with their scientific definition. High
density and/or partially metamorphosed limestones, especially those capable of
taking a polish, are oftentimes included in the marble group, because they
appear, behave and perform more similarly to marble than to limestone. Most
igneous rocks, such as gabbro, diabase, anorthosite, sodalite, gneiss, basalt,
and many others are included in the granite group because they behave and
perform similarly to granite. There are even a few non-igneous rocks (e.g.
silicate-based conglomerates) that are commercially grouped with granites.
Therefore, if you purchase a Crema Marfil "marble" vanity top, don't
be surprised if your geologist friend visits your home and insists that it is
limestone, because scientifically it is. Likewise, don't be surprised of the
same geologist friend informs you that your Paradisio "granite" bar
top and your Absolute Black "granite" kitchen island are really
gneiss and gabbro respectively, because scientifically that's what they
are. The key is performance. If a rock
is sold within the granite group, the rock should be expected to have
performance in that application that is similar, or in some cases superior, to
that of a true granite.
It
is a fundamental position of the Marble Institute of America that there exists
no such thing as a "bad stone". There do exist however, inappropriate
selections for a given application, and also unrealistic expectations of a
given stone type in a specific application. The informed selection of natural
stone products is also influenced by the tastes of the end user. To some,
natural wear, etching, or weathering bring about a hidden charm, or natural
"patina" as the stone displays signs of its yielding to the forces
encountered in its service. To others, the only acceptable performance is for
the stone to maintain its pristine, "as new" look for the entire
duration of its service life. Selections of natural stone types are available
to satisfy both users, but the proper research must be completed to assure that
the selected stone will perform in service with the desired behavior.
Granite
An excellent choice for kitchen countertops, floors, and
other heavily used surfaces
Exact
and current extraction figures are not available, as data collection from many
countries is difficult. Statistics from various sources indicate that the
granite quarried in the countries of China, India, and Brazil
comprises approximately 2/3 of the granite used worldwide. There are granite
quarries in operation in dozens of countries, and it is one of the most popular
natural stones on the market. New granite resources are continually being
located and developed throughout the world.
Granite
has long enjoyed use as an exterior cladding and pavement material, and its
inherent strength, abrasion resistance and superior weathering durability are
likely to keep it one of the preeminent material selections available to
today's architects. Granite has also been employed as the traditional material
for municipal curbs, where its strength and durability have been documented
with decades of vehicular abuse. In the northern climates where snow melting
chemicals are used heavily, granite has resisted the attack of these caustic
agents.
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| Being
one of the hardest of the dimension stone types, granite was historically
avoided by the smaller, local stone fabricating shops, who favored marbles and
limestones due to their easier working properties. A recent boom in the supply
of affordable machinery and abrasives technologies eliminated these previous
difficulties in fabrication. The use of granite has skyrocketed in residential
interior applications as a result. Available in a striking array of colors,
granite's durability, longevity, and economy make it ideal for kitchen
countertops and other heavily used surfaces, including table tops and floors.
Some
synthetic surfaces scratch easily, while the hardness of the minerals
comprising most granites surpasses that of the utensils that are used on them,
resulting in excellent scratch resistance. Granite is typically heat resistant
up to temperatures of ±250°C (±480°F), although direct application of localized
heat sources is discouraged, since strong thermal gradients within the stone
can initiate cracking. Studies of bacteria retention on common countertop
surfaces have proven granite to be superior to the majority of surfaces
employed for that purposes (Ref: MIA Technical Bulletins).
Absorption
rates (% water, by weight) of stones in this group range from 0.05% to 0.40%,
indicating that the available pore volume capable of harboring a staining agent
is very slight. Impregnating repellents are sometimes used to further reduce
the stain resistance of these materials.
Marble, Onyx, & Serpentine
Ideal for foyers, bathrooms, floors and hearths
Marble is a metamorphic rock found in the mountainous regions of most countries
of the world. Marble quarried in India, China, Italy, and Spain represents
the majority of marble, in terms of volume, that is utilized worldwide. Because
of its beauty and elegance, marble is a popular choice for countertops, floors,
foyers, fireplace facings and hearths, walls, and windowsills.
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Marble
with its inherent warmth, adds a sophisticated element to the area in which it
is installed. Its naturally random appearance, engineering characteristics, and
ease of maintenance makes it a premium choice for floors, wall claddings, table
tops, wainscot, floors, and vanity tops. Many marbles are well suited for wet
area application, which extends the versatility of this material to include tub
decks and showers.
The
calcite crystal is the basic building block of true marbles. The calcite
crystal is vulnerable to attack by mild acids, including those commonly found
in kitchen and bar settings. The user selecting marble for these applications
should be aware of, and accepting of the maintenance and patina that is to be
anticipated with this combination. Acid rain and other weathering elements can
also affect exterior marble installations, and exterior applications are
generally limited to white marbles, with some exceptions.
Often
mistaken for marble is serpentine, which is actually magnesium-silicate based
as opposed to calcite based. As a result of the different mineralogy and whole
rock chemistry of serpentine, it exhibits greater acid resistance and abrasion
resistance than does a true marble. These properties make serpentine a common
choice for both kitchen counter and exterior application. |
Onyx
is often confused with marbles, yet it is a significantly different rock type.
Onyx is a sedimentary rock, formed as stalactites and stalagmites in cave
interiors. This formation method results in the cryptocrystalline construction
of the rock fabric, and it is the size and uniformity of these crystals that
contribute to the classic translucent property of most onyx varieties. While
vulnerable to chemical and abrasive attack, the decorative appeal of onyx is
perhaps unsurpassed by any other material.
Sandstone and Quartzite
Exploring the "quartz-based" stones
The term sandstone refers to the sand sized (0.06 to 2.0 mm) clasts that are
cemented together by other agents. Therefore, sandstone could be of any
mineralogy, but the overwhelming majority of sandstones on the market are
quartz-based.
The
durability and performance of sandstone is not as greatly influenced by the
sand sized particles, as it is influenced by the cementing agent that binds
these particles together. Many types of sandstone are used in cubic sections as
sills, coping, water tables and other exterior features. Exterior cladding is
also a common application, although this stone variety is typically used in
thicker sections than other stone types due to lower bending strengths. While
sandstone has been used in both countertop and shower lining applications, the
varieties that are suitable for these installations are limited.
Quartzite
is a metamorphic rock that is formed from sandstone. Quartzite can be of exceptional
strength, density, and hardness. The strength, abrasion resistance, and
weathering durability of this rock type expand its application possibilities to
include most any of the common uses for natural, dimension stone.
Slate and Soapstone
Versatile, Chemically Resistant Materials
A traditional use of both these materials was the laboratory table top in
chemistry labs. That application alone should serve as a great testimonial to
the chemical resistance of the materials.
Being
of the softer varieties of dimension stone types, neither of these materials is
known for particularly high abrasion or scratch resistance, yet they are both
used a flooring and countertop products.
Soapstone
is highly heat resistant, and has been used in fireplace surrounds frequently
to take advantage of this property.
Slate,
being of laminar construction, has the ability to be processed into thin sheets
and still maintain serviceable strength and rigidity. This property makes slate
the only dimension stone having been used for blackboards and roofing shingles.
It was also traditionally used as the cloth-covered playing surface of
billiards tables.
Travertine, Limestone, and Dolomitic Limestone
An earthy appeal, indoors and out
Limestone deposits exist in all continents of the earth. Despite the common and
traditional reference to "travertine marble", travertine is really a
type of limestone. It is actually the terrestrial (land) formed
version of
limestone, as opposed to the marine based formations of many other limestone varieties.
Featuring
their soft earth tones, decorators integrating these stones into their design
have great flexibility in selecting complimentary colors for other interior
elements.
Many
varieties of both materials have enjoyed a successful history of exterior
application, and some of the most prominent government and financial
institutions worldwide proudly display limestone as their exterior cladding.
Despite the popularity of exterior vertical limestone applications, the number
of limestone varieties with successful history in exterior paving applications,
particularly in freeze/thaw environments, is relatively limited.
Since
these stones are some of the softer varieties of natural stone materials, they
have long been a popular choice for intricately carved features and moldings,
as well as statuary.
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| Limestone
and travertine, like marble, are of a calcium carbonate base, and as such, are
vulnerable to alteration by exposure to mild acids. A wide variety of stones
are included in this group, and absorption varies from slight (<1%) to high
(>10%). The combination of acid sensitivity and absorption limit the number
of varieties that are suitable for countertop applications, and the user of
limestone countertops should be well educated in its properties to accurately
anticipate its behavior in service.
Another
form of limestone exists, which is dolomite. Dolomite is based on the dual
carbonate of calcium-magnesium carbonate, and the properties of this stone are
influenced by this difference in composition. Dolomites generally have higher
densities, lower absorptions, greater compressive and bending strengths, and
higher abrasion resistance than the calcium carbonate based limestones. These
property differences offer some application choices for dolomites where other
limestone varieties are marginal or unsuitable performers. |
