Stone is a natural beauty with a variety of types and characteristics. It’s popular today in residential and commercial settings, as a result of its durability and maintenance. It’s essential to remember however, that different stones, although similar in appearance, may have different characteristics, and as a result have varying tolerances. It’s important to recognize what stone you have in your home and determine appropriate maintenance procedures, and precautions in order that it remain beautiful and functional.
The Stone Factory
The Earth is a stone-making factory. The Earth’s surface consists of huge plates which float on the molten mantle underneath. The Pacific plate lies underneath the Pacific Ocean and carries not only the ocean itself, but all the Pacific Islands including the Hawaiian chain. The North American plate carries not just North America, but also half the Atlantic Ocean. The other half of the Atlantic is part of the Eurasian and African plates. At the middle of the Atlantic Ocean is the Mid-Atlantic Ridge, which is a “spreading center.”
When these plates collide or separate, geology happens. Millions of years ago, the plates that now carry Africa, Europe, Asia, and North and South America had all combined to form one huge super continent. When they drifted apart, the Atlantic Ocean was born, and it is still widening at the rate of two-tofour centimeters a year. At the same time, the Pacific Ocean is slowly narrowing, dipping under North America and Asia, as the Atlantic and Pacific Oceans move towards each other.
The Earth is a constantly changing, living system. As earth’s plates continue their ongoing collisions, ancient sea bottoms are often thrust miles into the air. The summit of Mt. Everest is marine limestone, pushed more than seven miles into the air – so far – by the collision of India with the Asian continent.
Marble has been valued for thousands of years for its rich palette of beautiful colours and is perfect pretty much anywhere in the house, especially for foyers, fireplaces and bathroom walls, floors and vanities. Marble is usually polished to a mirror-like shine and runs the colour gamut from white and muted beiges to browns, rich reds and greens.
True marbles are formed from limestone or dolomite that has undergone enough heat and pressure to metamorphose into a crystalline structure. This metamorphosis takes place when temperatures in excess of 1800ºF are generated by the weight of overlying material, pressure from crustal collisions and heat from the earth’s core. True marbles are generally white or whitish, sometimes translucent, with some veining or colour provided by other minerals present during the process of metamorphosis. Foreign substances often entered the stone during this process, creating an infinite variety of colours, asters, and veining. Marble is a lot like snowflakes, no two pieces are exactly alike. Marble also has a variety of densities, many are lighter than granite, while others may be similar. Most marble has a higher absorption rate and lower abrasion resistance compared to granite.
Breccias, or brecciated marbles, such as Breccia Oniciata or Breche Nouvelle, are stones which have been broken up by earth movement, landslides and cave-ins, and recemented with various dissolved minerals such as silicates, resulting in the characteristic “broken” appearance. Most green marbles are technically not marble, but serpentinites, or serpentines, as they are more commonly called. These include the “jades” from Taiwan, and the very hard Verde Antique from Vermont. Because they are chemically more closely related to basalt and other mantle rocks, they aren’t subject to etching and tend to be a bit harder than other stones generally classified as marble. Kitchen use should be considered carefully, due to oil absorption. When polished, it requires high level of maintenance when used as flooring in high traffic areas.
Marbles range in hardness from 4-5 on the tenpoint MOHS scale (diamonds are 10; granites are ±7).
Slate is a fine grained, crystalline rock derived from sediments of clay and fine silt which were deposited on ancient sea bottoms. Superimposed materials gradually consolidated the sedimentary particles into bedded deposits of shale. Mountain building forces subsequently folded, crumpled, and compressed the shale.
At the same time, intense heat and pressure changed the original clays into new minerals such as mica, chlorite, and quartz. By such mechanical and chemical processes bedded clays were transformed, or metamorphosed, into slate, whole geologic ages being consumed in the process. Slates vary in composition, structure, and durability because the degree to which their determinant minerals have been altered is neither uniform nor consistent.
The most economically important slate deposits in this country lie in the MidAtlantic and Northeastern states transversed by or bordering on the Appalachian Mountain chain. Variations in local chemistry and conditions under which the slate was formed have produced a wide range of colours and qualities and ultimately determine the character of the slate found in these areas. Slate is available in a variety of colours. The most common are grey, bluegrey, black, various shades of green, deep purple, brick red, and mottled varieties. The presence of carbonaceous matter, derived from the decay of marine organisms on ancient sea floors, gives rise to the black coloured slates. Compounds of iron generate the red, purple, and green coloured slates.
Slate is very durable in most situations but the quality may varies. Although slate is typically rustic in appearance, it is a soft stone, which may easily be scratched. Wear is typically taken on the peaks or high spots of the textured surface. Slate characteristics vary with source. Domestic is typically black, green or mottled and is relatively dense. Imported slates are available with wide ranging physical characteristics and suitability.
Limestone has been used as exterior building cladding since time immemorial. Though most of it is gone, looted to build other structures, the Great Pyramids in Egypt were originally clad in smooth, perhaps even polished, limestone, as are many of the great cathedrals of Europe, such as Notre Dame in Paris. Sadly, many of these have fallen victim to a more modern menace, the acids found in polluted air, which have actually dissolved the surfaces and deteriorated the stone.
Limestone are sedimentary rocks that are formed at the bottoms of lakes and seas, as silt and organic matter settle to the bottom. The organic matter may be plants and marine shells or skeletons, which are preserved as fossils and serve as a guide to the age of the stone. As more and more layers build up over thousands and millions of years, adding more and more weight, the heat and pressure cause chemical reactions to take place to lithify the sediments into solid stone.
Though limestone is primarily calcareous – composed of the mineral compound calcium carbonate – other minerals present in the sediments produce various colours. Many of the limestone were formed during the Jurassic period, which makes them 140-190 million years old. Heat and pressure have a dramatic effect on the density and appearance of limestone. Beaumaniére and Gascogne, for example, are less hard than Rojo Alicante, New Pink, and Crema Marfil. Those latter three stones will take a high polish, and are usually classified commercially as marble.
Limestone is still being born on ocean floors. If you’ve watched any of the Jacques Cousteau specials, or the specials on exploring the Titanic’s wreckage, you saw the constant downward flow of particles and sediments in the process of forming layers that will someday lithify as limestone. As the Titanic continues to deteriorate and rust away, it will contribute iron and other minerals to that constant deposit, and give that vein of future limestone a large burst of colour, primarily reddish or yellowish. All stone is not alike.
Limestone is light in density and has a higher absorption rate and lower abrasion resistance than most marble and all granites. Limestone will require more time consuming maintenance than harder stones. Beware of limestone for high stress areas.
Limestone range in hardness from 3-7 on the tenpoint MOHS scale (diamonds are 10; granites are ±7).
Granite is an ingenous rock which means that at one time during its development, it was melted like volcanic lava. Unlike lava, however, this melted (or molten) rock never reached the surface. It remained trapped inside, where it slowly cooled and crystallized, resulting in a very uniform, speckled stone that ranges in colour from black and gray tones to pinks, browns, and reds. It is formed deep beneath the earth’s crust, and rises from the molten magma, forming at temperatures in the thousands of degrees.
Many of the oldest stones on earth come from the ancient cores of the continents, which probably formed and hardened as the earth cooled over four billion years ago. Dakota Mahogany granite from North America and Juperanas granite, from South America, Africa, and India, are among these truly ancient stones. Other, younger granites have formed under rising mountain ranges like the Sierra, welling up under the local “country” rock as plutons or domes. Sierra White and Academy Black came up under the future Sierra Nevada as the dinosaurs were dying out, 140 million years ago. New granites are forming even today, far beneath the Alps and the Himalayas, which are still rising.
As with any natural stone, the colour, pattern, veining and shading of granite will vary from tile to tile – sometimes even within a single tile. These variations are completely natural and help to create an overall appearance that is truly unique. Certain granites contain iridescent quartz crystals that reflect light and seem to shimmer and sparkle. They may appear different from different directions in relation to the available light in the area.
Granite is especially popular for use as flooring or countertops. Unlike marble, granite is a relatively hard, non- porous, natural stone. It is relatively resistant to most household kitchen products and handles heat extremely well. Some granite however, can be stained by some liquids or darkened by oils. It is therefore best to practice preventative maintenance procedures, seen on following pages. Care involves regular cleaning with warm, soapy water.
In a study conducted by the Hospitality Institute of Technology and Management, measuring the bacteria resistance capacity of common countertop materials, granite came second to stainless steel.
Care does need to be taken, however when selecting any natural stone for a high-traffic area or kitchen countertop which might be subject to etching substances. Like any stone, it should be sealed to prevent stains from penetrating into the stone.
The finish of a stone contributes a great deal to its beauty, its durability and its use in a variety of applications. Here’s some information on finishes:
Polished: Large machines progressively grind the stone to a specular, mirror-like finish.
Honed: This less formal, softer-appearing stone has a matte or satin finish.
Flamed: Blowtorch-strength heat is applied to the surface to create a deeply textured surface ideal for outside use.
Tumbled: This stone is tumbled with sand, pebbles or steel bearings to create a weathered, aged finish.
Filled: When travertine is used for tiles and slabs, it is usually “filled”—that is, the surface holes are filled with cement or sometimes epoxy, and then polished or honed like any other stone tiles or slabs.
Commonly Asked Questions
What are hard and soft stones?
Natural Stone is divided into hard and soft stone categories.
Granite, Gneiss, Quartite, Porphyr
Limestone, such as Travertine, Botticino, Trani and all varieties of marble.
What is tumbled marble?
Tumbled marble is marble tiles that are actually tumbled together in large drums. The end effect, being a worn appearance and soft feel of the tiles is the result of the tiles rubbing together within the drum.
What is the MOHS scale?
Hardness of a crystalline solid, with respect to abrasion often refers to the ability of one mineral to scratch or abrade another, this is called Mohs’ hardness.
Mineral hardness is the most important factor in the lapidary working of minerals and rocks. A mineral can be scratched by an abrasive of equal or greater hardness than itself.
Mohs’ scale is a scale that measures mineral hardness… relative mineral hardness not absolute. Mohs’ scale is an ascending list of 10 common minerals that increase in hardness. As a result the scale cannot be used directly to qualitatively define the actual hardness of a mineral. It was originally designed by Fredrick Mohs in the early 19th century.
What is Soapstone?
Soapstone is one of the softest materials, which is composed of the mineral talc. For this reason it makes an excellent carving material and can be found on fireplace surrounds and hearths. It is treated like marble.
Proper preventive and routine maintenance is required to keep stone in good condition.
- Provide floor mats or area rugs inside and outside entrances to prevent dirt,
grit, acidic rain water, rock salt, and ice melt from being dragged across
stone floor. Be sure underside of mats and rugs have nonslip surface.
- Keep floor dirt and dust free. Dirt particles canscratch the stone and dust
makes the floor slippery. High traffic areas should be cleaned more often.
Walkways outside of entrances should be swept to prevent dirt particles from entering building.
- Clean spills immediately before they stain. Keep water from spilling on the surface, especially around drinking fountains and planters
- Keep metal objects away from marble. A metal can could rust and stain marble if exposed to moisture.
- Practice regular and appropriate maintenance procedures.
- A good rule is to not use anything on stone that you wouldn’t use on your hands.
- Do not cut directly on your stone counter top. This can cause deep and permanent scratches to appear.
- Avoid directly dropping heavy or sharp objects on your counters.
- Avoid flame or applying heat directly to your stone counter tops.
Soft stone can be damaged by acids. With polished surfaces, permanent dull areas can develop if exposed. Materials such as wine, fruit juice, cola, vinegar, lemon juice should be kept away from marble, as well as scouring powders which contain abrasives.
Marble isn’t quite as worry-free as granite. It has a softer, less stain-resistant surface than granite and should be treated like a fine piece of wood. Spills should be wiped up immediately, coasters should be placed under beverages to avoid staining and etching. Marble is especially susceptible to damage from citric acids, alcohol’s, and oils. Routine maintenance should include dry dusting with a soft cloth as needed.
- Dust mop frequently with clean, nontreated cloths/mops. Dirt and grit are abrasive and scratch stone. Do not use vacuum cleaners on stone floors. The plastic attachments and wheels will scratch the surface. Higher traffic areas need to be cleaned more often.
- Clean stone with appropriate cleansers. Do not use vinegar, lemon juice, cleaners that contain acid, or abrasive cleaners. Any cleaning product should be tested in an inconspicuous location to ensure compatibility with the material and to avoid damage.