Non-metallic inclusions, the presence of which defines purity of steel, are classified by chemical and mineralogical content, by stability and by origin. By chemical content non-metallic inclusions are divided into the following groups:
#sulfides (simple — FeS, MnS, AlS, CaS, MgS, ZrS and others; compound — FeS·FeO, MnS·MnO and others);
#nitrides (simple — ZrN, TiN, AlN, CeN and others; compound — Nb(C, N), V(c, N) and others), which can be found in alloyed steel and has strong nitride-generative elements in its content: titanium, aluminium, vanadium, cerium and others;
#silicates
#oxides (simple — FeO, MnO, CrO, SiO, AlO, TiO and others; compound — FeO·FeO, FeO·AlO, FeO·CrO, MgO·AlO, 2FeO·SiO and others;
The majority of inclusions in metals are oxides and sulfides since the content of phosphorus is very small. 
Silicates are very detrimental to steels, especially if it has to undergo heat treatment at a later stage.
Usually nitrides are present in special steels that contain an element with a high affinity to nitrogen.
By mineralogical content, oxygen inclusions divide into the following main groups:
* Free oxides — FeO, MnO, CrO, SiO (quartz), AlO (corundum) and others
* Spinels — compound oxides formed by bi and trivalent elements
Ferrites, chromites and aluminates are in this group.
*silicates, which are present in steel like a glass  or SiO with admixture of iron, manganese, chromium, aluminium and tungsten oxides and also crystalline silicates. Silicates are the biggest group among non-metallic inclusions. In liquid steel non-metallic inclusions are in solid or liquid condition. It depends on the melting temperature.
By stability, non-metallic inclusions are either stable or unstable. Unstable inclusions are those that dissolve in dilute acids (less than 10%concentration). Unstable inclusions are iron and manganese sulfides and also some free oxides.
Present-day levels of steel production allow to move off from the metal different inclusions. However, in general the content of inclusions in different steels varies within wide limits and has a big influence on the metal properties.