Rocks on the surface of the Earth are weathering, i.e.. mechanical and chemical deterioration due to the action of weather conditions. The material formed as a result of weathering may remain at the site of its origin or it may be carried mainly by the waters of streams and rivers in the form of a suspension or various sizes of crumbs. Sedimentary rocks are formed by the deposition of this material and the remains of plants and animals in water bodies or on land.
Weathering processes can sometimes contribute to the formation of new minerals. Some gemstones are also made in this way, like malachite, azurite 'turquoise and chrysoprase. Malachite and azurite are formed from sulphide ores as products of copper weathering, turquoise is formed in the processes of weathering certain igneous rocks and the adjacent copper ore. Nickel-bearing chrysoprase is the end product of serpentine rock weathering, consisting of nickel.
It is much easier to extract precious stones from weathered igneous rocks than from primary rocks. An example is the occurrence of diamonds in South Africa, where it is not difficult to extract them from the so-called. yellow earth (yellow ground), which is a product of weathered bluish kimberlite rock (blue ground). More than once, the crushed original rock is exposed to the influence of weather conditions, to speed up the airing process.
In the colloquial language, the word rock is usually associated with the concept of compacted and hard material, in geology and petrography, however, i.e.. in rock science, rocks also include loose gravel and sand. They consist mainly of the minerals and fragments of primary rocks most resistant to weathering, because a significant part of the primary minerals undergoes mechanical destruction and chemical transformations. Only the most resistant of them remain, like quartz, the content of which in gravel and sands often exceeds 90%. Sometimes these rocks also contain precious stones. Originally they were found in other rocks, after weathering which they were transported by rivers and deposited with other minerals resistant to weathering. W wyniku długotrwałego transportu rzecznego kamienie te ulegają nieraz obtoczeniu. Ponieważ gęstość kamieni szlachetnych jest z reguły wyższa niż pospolitych minerałów skałotwórczych, e.g.. quartz, ich osadzanie następuje zwykle tam, gdzie zmniejsza się szybkość wody płynącej, w niektórych miejscach w ilościach pozwalających na ich eksploatację. This creates secondary crumb deposits in the form of distinct levels. The importance of these deposits is often greater than that of the primary deposits. African deposits are an example of such deposits, Brazilian, Indian d other diamond deposits, deposits of rubies in Burma or sapphires in Siam and Ceylon. They occur both in the beds of modern rivers, as well as in the sediments of river terraces.
The extraction of precious stones from this type of crumb deposit, called alluvial, usually does not present any major difficulties; nie ma zwłaszcza obawy uszkodzenia kamieni, co nieraz zdarza się przy wydobywaniu ich ze złóż pierwotnych. Na ogół chodzi tylko o dostatecznie dużą ilość kamieni, aby eksploatacja była opłacalna. Furthermore, stones from secondary deposits are often of high quality, because crystals containing nodes and cracks break down into smaller ones during water transport, and those that have survived usually have no disadvantages, nor internal defects.
Gravel and sand containing precious stones are not always found on the surface. They are often covered with worthless rocks, after their removal, the levels containing precious stones may only be exploited. This is often done manually or with mechanical excavators and dredgers, sometimes even tanks.
Loose deposits of gravel and sand can be cemented and transformed into hard rocks. The various processes that cause this are generally called diagenesis. Conglomerates form from the gravel (konglomeraty), and the sands are formed into sandstones. Such chemical compounds can be the cement that cement the individual components of these rocks, like silica, carbonates, especially calcium carbonate, iron oxides, clay substances, etc.. To extract gemstones from such compact rocks, crush them. The methods used for this purpose are analogous to those used in gold mining. A strong current of pressurized water is commonly used, which breaks and crushes the agglomerates into gravel, and sandstones to sands, which are sieved by mechanical devices. Thousands of cubic meters of rock are thus crushed in diamond bearing areas.
The described sedimentary rocks belong to the group of rocks called crumb rocks, clastic or rocks of mechanical origin. Apart from them, there are also other sedimentary rocks, resulting from chemical processes or from organic debris. Examples of rocks of chemical origin are silica or salt deposits, and the most important rocks of organic origin (organogenic) there are limestones. Sedimentary rocks also include dolomites formed from calcareous sediments as a result of the action of magnesium-containing sea water or hot magnesium solutions on them.. The substances of organic origin are classified as gemstones pearls, corals, amber and jet.
A characteristic feature of sedimentary rocks is their stratification related to the conditions of their formation. They are formed mainly by deposition in the aquatic environment, most often by sea. In most sedimentary rocks, their components are cemented with a binder.