Natural Stone Quality
Sooner or later all stone will crumble and disintegrate. Moreover, it is important that the stone chosen is suitable for the intended purpose - and that it is handled it correctly. Limestone and marble, both soft rock types, are soluble even when exposed to weaker acids. A polished marble facade in the typical Norwegian coastal climate will have a low life expectancy and rapidly become dull. Similarly, a limestone floor in the entrance to a shopping mall will deteriorate quickly, as by the quartz sand and salt carried on winter shoes abrade its surface on a surly winter day. Although the same rocks can be very durable in southern Europe, we have specific climate disadvantages up in the North that are unavoidable.
However, even hard granites experience problems. Under long term exposure to humidity, iron can precipitate from the ferrous minerals in the stone and then oxidize on the surface to form rust stains - an unsightly discoloration. Furthermore, even granites can crack and flake if exposed to salts, whether it be street de-icing salt, or alkaline salts present in the mortar. On the whole, salt is one of worst enemies of natural stone that may exhibit varying degrees of porosity. Solutions of salt and water are transported into pores, and when the water evaporates, the salt crystallizes and expands thereby cracking the stone. Another 'enemy' comes from within - some minerals easily dissolve and if they contain iron, as mentioned already, rust can accrue on the surface of the stone. Examples of stones that may exhibit this include olivine, pyrite and pyrrhotite.
In addition, Norwegian Mineral Industry (Norsk Bergindustri) has prepared guidelines on the use of natural stone for indoor environment and outdoor setting.
Factors to be considered before production begins
Natural stone must always be extracted in whole, large pieces. The many naturally-occurring cracks, faults, fissures, must be assessed in order achieve the optimal size of block. The amount of exposed stone must be large. Because natural stone is used for architectural purposes, aesthetic quality is perhaps the most important quality criteria. Simply put, the stone must be nice to look at, and price will vary accordingly. Therefore, the rarest and most beautiful stone types are more valuable than ordinary "gray stones".
Natural stone must homogeneous throughout the deposit, without spots, veins and other irregularities that will reduce production. Unfortunately Mother Earth is full of 'beauty' marks. As mentioned above, natural stones must have a strength and durability that meet the requirements for various uses. In terms of durability, the presence of "unwanted" minerals that might lead to discolouration or deterioration of the stone.
Another important issue, are those characteristics of the stone which influence positively or negatively on the production costs, and determine the feasibility of production. Some Norwegian granites have so good cleavage properties that wedge and split just by perforating with a line of drill holes, thus making it less expensive to quarry than those granites with "heavy" and poor cleavage. Most natural stones tend to have a preferred sawing plane; that is in the direction that you getting the nicest looking slabs. It is important that sawing plane is against the bedding plane, the main cleavage - it will be much like to sawing wood against its grain.
Disse kvalitetskravene gjelder for de fleste markeder der naturstein selges. En steinforekomst i Norge bør helst være så spesiell at prisnivået blir høyt. Alternativt må steinen være ekstremt billig å ta ut, det vil si at forekomstene er svært massive (lite sprekker), homogene, store og lett tilgjengelige. Noe annerledes stiller det seg med stein som brytes for lokale og regionale markeder til muring, forblending og platting. Her er viktigste kriterium at uttakskostnadene er lave - utseendet spiller mindre rolle.
These quality requirements apply to most markets where natural stone is sold. In order achieve a highest price possible, a stone from Norway should ideally be unique. On the other hand, if the stone is very cheap to quarry; that is, it should be extremely intact (only small cracks), homogeneous, large and easily accessible. Other factors must be in place for stone that is quarried for local and regional markets to brick work, veneers and stone terracing. In this case, most important criterion is that the extraction costs are low, and appearance is lower priority.