Since we are interested in beige limestones, let's consider the obvious signs of poor quality stone of this type of rock.
The main signs of low-quality stone are:
Open (not filled with minerals) sutural seams
A characteristic feature of such defects is the tiny voids along the seams, formed by the removal of clay and iron oxides (fig. 1). Such defects are clearly identified when running a fingernail across the surface of the stone. It should be noted that it is necessary to inspect not only the face surface of the tiles but also to carefully examine their side surfaces (fig. 2), as unscrupulous manufacturers sometimes camouflage cracks and open sutural seams on the face surface of the tiles, making it difficult for a non-specialist to detect such formations.
Suture seams filled with weather-resistant minerals (Fig. 2, usually calcite) do not get discolored during stone processing at the factory, and the surface polish is preserved. Such stone can be used for cladding.
🔍Fig. 1: Open, non-mineralized sutural seams in marbled limestone.
🔍Fig. 2: Open sutural joints in marbled Jurassic limestone "Jura Beige" (Germany).
Caverns and stone lighting
In those areas of the stone where the cutting plane coincides with the plane of the sutural joint, areas of surface destruction in the form of caverns and shells are formed on the stone. In such areas, the stone does not accept polishing and cannot be treated (Fig. 3). Caverns and lightening are another sign of poor stone quality. Such stone should not be used in stone cladding.
🔍Fig. 3: Caverns, shells, and lighting on Jurassic marble "Jura Beige" (Germany) of poor quality.
Calcite veins
For some varieties of marbled limestones of beige and brown color, the presence of calcite veins is characteristic. Unlike sutural seams and cracks, such formations in the stone are not accompanied by steps or depressions. A probe or profilometer will not detect such formations. However, veins of any color can be easily identified visually. Typically, calcite veins are made of white or dark gray coarse-grained calcite (Fig. 4) and very rarely have pink tones of coloration.
By themselves, as ultrasound studies show, calcite veins do not accompany a decrease in the strength properties of the stone, and only change the optics and texture of the stone. There is an opinion that these cracks manifest negatively as the weakest element of the system when processing Jurassic marble, especially in construction site conditions.
What does this mean? When cutting the slab to the required size, the disc reaches the calcite vein and provokes a break along the so-called "crack," thus making the Jurassic stone slab unsuitable for use.
🔍Fig. 4: Technological destructions of the facade slab made of Jurassic marble during cutting at the construction site.
🔍Fig. 5: Fragments of slab pieces that allegedly broke along the calcite vein.
After numerous checks of the cuts (Fig. 6) on the selected slabs of Jurassic limestone with a significant amount of calcite veins, this issue was not confirmed.
🔍Fig. 6: The passage of the cutting disc through the calcite vein.
Large inclusions of shells
In marbled limestones of beige and gray colors, fossilized shells are quite often found. This, for example, is a distinctive feature of Jurassic marble, which carries the imprints of ancient inhabitants of the Jurassic period. Such fossils usually represent areas of recrystallized calcite, sometimes of a darker color.
These inclusions may not affect the strength properties of the stone, but they can significantly worsen its durability. Here, it is necessary to closely examine the structure of the calcite grains that make up the fossils. If the calcite is coarse-grained, it will inevitably chip away over time (Fig. 7 above); if the inclusion material does not differ in structure from the host rock, the durability of such a facing slab will not be compromised (Fig. 7).
For stone lovers, the inclusion of fossils is a positive factor, as it emphasizes the natural nature of limestone and its individuality. For consumers who are not very familiar with rocks, such inclusions may seem like a dissonance, supposedly disrupting the pattern of the stone. Whether to include such slabs in stone cladding or to reject them depends on the consumer's personal attitude towards them.
🔍Fig. 7: Inclusions of fossil mollusk shells in beige limestones.
At the top is limestone from North Africa. The inclusion of a fossilized shell is short-lived.
At the bottom is Jurassic limestone "Jura Beige" (Germany). The inclusion of a fossilized shell does not affect the durability of the slab.
Inclusion of sulfides
In any limestones, including those with beige tones, dark brown speckles may be found (Fig. 8). These small spots, usually no more than a few millimeters in diameter, represent limonite (limonite [Fe2O3◦nH2O]). Limonite is a collective term for natural mineral aggregates that consist of mixtures of trivalent iron hydroxides. The iron oxides in the stone formed due to the oxidation (tens of millions of years ago) of the mineral - pyrite (FeS2), which was contained in the rock from which blocks were later extracted and cladding tiles were made.
The brown-toned edges of iron hydroxides around such inclusions can exceed the diameter of the pyrite inclusion by 3-4 times (not more). Limonite is a mineral that is stable in atmospheric conditions, and the spots formed by it will not change during the operation of the stone. Since such inclusions contrast in color with the overall background, the presence of brown spots reduces the decorative quality of the stone in these areas. There is a common belief that these spots, for example in the texture of Jurassic marble, are rust, which, when used in an external environment, will inevitably be dissolved by rainwater and lead to the formation of cavities and streaks.
From the above, it follows that limonites and their primary source - pyrite - should be distinguished. It is pyrite that forms rust and leads to the destruction of stone. This material is typically found in Jurassic limestone of gray-blue color in layers from the 1st to the 6th. These layers are deliberately not recommended by conscientious manufacturers for use in exteriors.
🔍Fig. 8: Inclusions of sulfides in beige limestone.
Geodes and secretions
In limestones, geodes and secretions are often found. A geode is a geological formation, usually spherical in shape, partially or entirely composed of cryptocrystalline or clearly crystalline mineral substance. In beige Jurassic limestones, geodes and secretions are typically made of calcite (fig. 9). Voids up to 10 cm in diameter are commonly referred to as geodes; if the size of the formation is larger, it is called a secretion.
The presence of geodes on the facing slab, if its size exceeds 20 mm, is considered a defect and such a slab is not used for cladding. If the void is smaller, it can be filled with a durable mastic that matches the color of the stone. In any case, the presence of such formations reduces both the decorative quality of the facing slab and its durability. It is not recommended to use slabs with geodes and secretions in stone cladding.
🔍Fig. 9: Geode in beige limestone.
Cracks in beige limestone
During the process of finishing work, incoming control of products is usually conducted. Jurassic stone slabs with cracks should not be used for cladding. However, there are exceptions. A crack in the slab may occur during careless installation or as a result of damage during transportation to the installation site.
It is necessary to be able to recognize a crack and timely reject such slabs. A crack is a disruption of the continuity of the rock. At the location of its occurrence, the color of the stone will inevitably change (fig. 10). If you notice straight thin stripes or strokes on the stone, carefully examine this area of the stone and investigate for the presence of a ledge. If there is a ledge, it is a crack; if there is no ledge, it is a vein.
🔍Fig. 9: Cracks in beige limestone.
The provided information helps to avoid the presence of obvious defects in stone cladding and will save you from many troubles when purchasing stone from an unscrupulous supplier.
It should be noted that when using stone in exterior cladding, external inspection is not sufficient. Laboratory testing of the material to determine its physical and mechanical properties is absolutely necessary. But that's another story.