Knotted limestone

Knotted limestone is a limestone that stands out because of its knot-like concretions and colored appearance. It is formed by diagenesis, i.e., compression and cementation, of lime muds deposited in seas. These lime muds deform into nodular structures, with the clay in the lime mud being partially deposited and compressed in the marginal zones of the nodules.
This process takes place at ocean depths of about 300 meters or more and results in the formation of nodular limestone. The clay mineral layers often completely enclose the carbonate accumulations in the rock and enhance the visual impression of these rocks. In red nodular limestones, the color is due to hematite. Many nodular limestones formed during the Devonian.

There are several theories on the formation of nodular limestone. One common explanation is diagenesis of lime muds, while other researchers cite metamorphic processes as the cause. Tectonic stress can cause planar textures of clay skins to form around the remaining lime accumulations.

Tuberous lime is often used for decorative purposes due to its structurally variable and colorful appearance. Depending on weather resistance, nodular limes can be used outdoors or indoors. In Europe, nodular limestones have been quarried in various regions for architectural, sculptural and craft purposes. For example, varieties from Italy were used in Bavaria primarily as columns and balustrades. In Italy, especially around Lake Garda, road curbs and flooring are also made from it. In Austria, Adnet nodular limestones from the province of Salzburg are of great importance for Gothic funerary sculpture.

It should be noted that nodular limestone is a specific type of limestone. Limestone in general is called sedimentary rock, which consists mainly of calcium carbonate (calcite and aragonite). Limestones can be of biogenic, chemically precipitated or clastic origin. They have great economic importance as a raw material for the construction industry and as a natural building stone. Limestone can also serve as a reservoir rock for crude oil and natural gas. The term "limestone" is used in a variety of contexts, including the building materials industry and the natural stone industry. However, there are differences in the use of the term in colloquial, technical, and scientific language.