Marble, granite and limestone: history and general information
Natural marbles are metamorphic rocks, occurring in a wide range of colours, consisting of fine to coarse-grained calcite or dolomite. They have been re-crystallised by natural processes in the earth, causing them to frequently develop a grainy, “sugary” mineral texture. Rocks which are now marbles may have formed originally from ancient coral reefs (in which case you would find fossil outlines of corals, sea shells and other forms of marine life within the rock), or from calcareous sediments (where you would expect to still see the original sediment’s mineral grains and bedding layers). Marbles which have undergone particularly high metamorphic pressure and temperature changes tend to lose their original sedimentary features, and become coarsely banded rocks. Many types of marble may be hundreds of millions of years old, and have been altered from their original state by a variety of natural earth processes (usually metamorphic changes involving great heat and pressure). These may cause the marbles to be folded, fractured, or veined with different minerals, or carry particular textures, streaks or colour patches which are indicative of the metamorphic processes which have changed them. Both the original sedimentary variation within the rock, and the subsequent metamorphic events that changed it, impose variations of colour, grain size, mineral pattern and mineral texture which are both totally natural and are in fact characteristic of the rock.
The term “marble” in industrial and commercial (as opposed to geological) terms applies to more or less any crystallised limestone capable of taking a polish or of being used for fine architectural or ornamental purposes.
Strengths and Weaknesses
Marble is a natural product and, as such, will vary in nature as the different batches of material are mined from the quarry. No two pieces of marble (and therefore no two fireplaces) can be exactly alike in colouration or the amount of veining. Fossils and veining are a normal and acceptable part of genuine marble. Marble also nearly always has holes in it (pores), which will be filled prior to polishing. (Filled marble is not a sign of inferior material – travertine, for example looks like Swiss cheese in its natural form).
Your marble surface or fireplace is relatively easy to care for but do be aware that tea, coffee and red-wine will stain and must be dealt with quite swiftly and would possibly require a specialist product. For the routine care and maintenance, however, a marble care kit is recommended. This comprises an anti-stain sealant, a cleaning agent and a marble polish. The care kit will keep your fireplace or marble surface in pristine condition over the long term. Montpellier Marble or your local stockist can advise you on suitable products for the care of your fireplace.
Granites are coarse-grained igneous rocks, solidified from molten lava, which have cooled and crystallised at great depth within the earth (some granites are metamorphic in origin, but these types are less important in the ornamental stone industry). Because they were cooled slowly at depth, granites and their individual mineral crystals have had time to grow large, so today we see them as coarse grained rocks with a granular, interlocking mineral texture. The deeper they were formed, the larger the size of the grains, making up the granite rock, and the more attractive they can be as ornamental stone samples. Granites are usually formed of grains of quartz, feldspar and mica, but often with other minerals in small quantities as well. This combination of pale and dark minerals gives the granite a characteristic “spotty” or mottled appearance. Because it contains a high percentage of quartz grains (which are hard and resistant), granite is normally a hard and durable rock type which weathers very slowly. For this reason, areas of granite often form resistant hills and mountains. This quality also makes granite a durable and long-lasting ornamental stone. After formation, granites (like all other rocks) are frequently subjected to deep-seated, earth moving pressures (e.g. mountain building, also called orogenesis), which can cause natural changes to the original granite rock. These changes may include new alignments of mineral grains, irregular colour changes, jointing, and mineral veining (e.g. with quartz and calcite), which adds to the natural visual variation in the rock.
Strengths and Weaknesses
Granite is a natural material, and as such in inherently variable in shade and colour. No guarantee can be given that the product will be uniform, or indeed the same colour as depicted in the showroom as samples.
Granite can be cared for in the same way as marble. It is less liable to stain than marble but care must still be taken. Wipe any spills immediately, especially acidic liquids such as fruit juice or wine. Avoid dropping anything onto the surface or resting on the granite surface as this can cause chipping and cracking. Specialist care kits can help preserve the condition of your granite over the long term.
In geological terms, a limestone is a sedimentary rock consisting mostly of calcium carbonate, usually in the form of the mineral calcite. Common accessory minerals include silica, feldspar, clays, pyrite and siderite. Limestones may be formed in a variety of ways, but many are highly fossiliferous and clearly represent ancient shell banks or coral reefs. Many examples of limestone may be hundreds of millions of years old, and can give a fascinating indication of general reef conditions at the time of formation (including types and sizes of shells, corals, etc.) A detailed study of ancient reef limestones will show many species of shell and coral which became extinct on earth millions of years ago. Because of their environment of formation, most limestones will show a naturally high degree of variation, in terms of colour, fossil content, mineral grains, etc. Like all rock types, limestones have often been subjected to natural earth stresses, resulting in features like jointing, which are sometimes filled by later veins of minerals like quartz and calcite. As with the original variations of colour and mineral texture, described above, these joints and veins are totally natural features, and indicate the history of formation of the limestone.
Strengths and Weaknesses
Limestone is a natural material, and as such, is inherently variable in shade and colour. No guarantee can be given that the product will be uniform, or indeed the same colour as depicted in the showroom or in samples.
Limestone is extremely porous and therefore liable to stain. Always ensure that limestone surfaces or fireplaces are sealed with a specialist product before use. The sealing process should be carried out at regular intervals, say twice a year; to ensure the limestone is protected. Always use specialist limestone cleaning products for stain removal or simple cleaning and maintenance. Montpellier Marble or your local stockist can advise on the best products to use.