Like granite, the term "marble" has both a geological and commercial definition. Geologically, marble is a "metamorphic rock consisting of fine to coarse-grained recrystallized calcite and/or dolomite." Commercially, the term marble is used to describe any crystallized carbonate rock including true marble and certain limestones (orthomarble) that is capable of taking a polish. Travertine and serpentine, while not true marbles, are usually included in the commercial definition of marble.
A geologic marble of pure calcite or dolomite would be white. Marbles are available in a vast array of colors, and the variety of colors is the result of impurities found in the marbles, such as iron oxides, carbonaceous minerals, mica, chlorite or silicate.
The Marble Institute of America defines 4 classes of marbles in descending order of soundness as A, B, C, & D. Research of a particular marble is necessary to determine its suitability for the application, particularly exterior applications. Marbles in the C & D soundness classifications have the most limitations in fabrication and application, yet many of the highly decorative marbles with exotic veining fall into these groups.
There is a wide variation of physical properties in the stones that make up the marble group. Generally, the flexural strength, compressive strength, and abrasion resistance are lower than what is typically found in the granite group. This may result in either reduced spans or increased thicknesses for cladding, or increased wear in high traffic areas. As long as the material is properly selected for the application, the performance should be satisfactory.
Some marbles exhibit hysteretic behavior after repeated heating and cooling cycles. This hysteresis is typically evident by a bowing of the marble panels, and can normally be eliminated if the panels are of sufficient thickness. Research of the particular marble should be done to determine its vulnerability to this phenomenon.
The Limestone Group
Limestone is a sedimentary rock consisting mainly of the mineral calcite (calcium carbonate) with or without dolomite (magnesium carbonate). The color of limestone is altered by the presence of impurities, which broaden the color spectrum of limestone to include white, brown, gray, buff, yellow, red, block, or mixtures of these colors.
Limestone, being of sedimentary origin, is usually quite anisotropic, or "directionally specific" in its behavior. This accounts for a pronounced "rift", or plane of easiest splitting, within most limestone types.
The flexural strength of limestone usually necessitates the use of thicker panels for cladding applications, where 3" or 4" thicknesses are not uncommon.
Due to high absorption and susceptibility to staining, Limestone is not generally used is applications where it comes into contact with soil. A limestone clad building utilizing another stone type for the exterior base course is traditionally an attractive solution to this limitation.
The composition of this stone type allows for the cutting of profiles by means of "planing". The use of a plane to shape the stone makes profiles pieces (e.g. cornice or moulding pieces) more economical than in other stone types.