Dolostone, also known as dolomite (not to be confused with the mineral dolomite), is a type of carbonate sedimentary rock consisting predominantly of dolomite [CaMg(CO₃)₂] and to a lesser extent of other carbonates, like calcite and aragonite. Like the mineral, the name derives from the French mineralogist Déodat Gratet de Dolomieu, after whom also the Dolomites of Italy, where the rock was discovered, take their name. Likewise limestone, dolostone forms as a result of carbonate precipitation from water, in particular dolomite, which precipitates following the reaction:

Ca²⁺ + Mg²⁺ + 2(CO₃)^2- ⇌ CaMg(CO₃)₂

The precipitation of dolomite from a water solution is controlled by temperature, pressure, and solubility of CO₂. Differently from limestone, which is produced by a wide range of organic and inorganic processes that occur at ambient conditions, the precipitation of primary dolomite directly from the water column is very rare and seems not to occur in present-day ocean, despite the fact that seawater is saturated in dolomite. Rather, most dolomitic rocks derive from the chemical replacement and cementation of original calcite and aragonite carbonate sediments by secondary dolomite. The process of dolomitization tends to produce rhombohedral crystals of dolomite, modifying in part or totally the texture of the original limestone. Some dolostones still preserve fossils or other allochems, while others only show a crystalline texture, consisting of recrystallized rhombohedrons of dolomite, which could either represent primary dolomite or completely dolomitized limestone. Dolomitization of carbonate sediments seems to happen mainly by two mechanisms: evaporation of seawater that produces gypsum-rich brines that migrate in the underlying carbonate sediment, reacting with calcite to produce dolomite (evaporative reflux) or interaction between meteoric and marine groundwater (mixing of seawater and meteoric water).