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The properties of stone, such as its weight, hardness, resistance to fire and to weather, have made it possible to build solid constructions from the static point of view, whose lifetime is measured in centuries but not in years. Consequently stone, over the centuries and up to the early 20th century, was preferred by builders to other materials, except in those places where it was impossible to find stone or too expensive to transport it from other production sites.
Subsequently, stone lost its pre-eminent place due to the high costs of quarrying, processing and transporting it, in addition to its high specific weight and, in final analysis, due to the high incidence on economy of stone buildings (resulting not from higher building costs, but from the thickness of stone walls, which occupy too much space in respect to the new technologies). All of this has weighed on the role of stone, which for centuries had retained its role as structural element par excellence, but which has come in our own day to occupy only a marginal place among the major building materials.
Today in fact, stone materials are utilized in secondary roles, while concrete, tile and steel have come to predominate in the structural function. Stone is utilized today basically as a material of completion and finish; as well as in the form of aggregate for road works, for the formation of concrete mix and as raw material for the production of binders. The predominant usage is thus no longer as paving stones, ashlars, or bearing pieces, but is now mainly for walls, flooring, facing, finishes in general or for conservative restoration. In other cases stone is used for molding or at most for the portion of a facade extending up to the first floor which, in more finished constructions, may be built of stone. More widely diffused instead is the use of stone cut in slabs for window frames, jamb stones, thresholds, steps, and this is due to its characteristics of hardness and practicality.
Stone materials may be classified as:
1. marble, (compact, resistant rock, suitable for polishing)
2. granite, (intrusive magma rocks)
3. stone (comprising rocks with highly varied chemical composition.)
4. travertine.
The commercial denominations of stone materials include various lithological types and are codified by UNI standards.
As concerns marble and granite the denomination is based on the hardness of the material, on which depend both its workability and its performance when installed. Ornamental rocks are included in the categories of marble, granite, travertine and stone. Under the heading marble are classified materials with Mohs hardness of 3-4, under the heading granite materials with Mohs hardness of 6-7. It may be deduced that, in addition to marble and granite in the strict sense, this category includes numerous other families of rocks.
Legislation on this subject is now undergoing transformation, since the European Standardizing Committee CEN/TC 246 (relevant to natural stones) has after 10 years almost completed its work program. The first standards have already been published, translated and adopted in the various nations of the EC, while the rest are scheduled to arrive soon.
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