Closely related to limestone is dolostone, a rock composed of the calcium-magnesium carbonate mineral dolomite [CaMg(CO3)2].
Although dolostone and limestone sometimes closely resemble one another, they can be easily distinguished by observing their reaction to dilute hydrochloric acid. When a drop of acid is placed on limestone, the reaction (fizzing) is obvious. However, unless dolostone is powdered, it does not visibly react to the acid.
Dolostone’s origins remain a subject of discussion among geologists. No marine organisms produce hard parts of dolomite, and the chemical precipitation of dolomite from seawater occurs only under conditions of unusual water chemistry in certain near-shore sites.
Yet it is abundant in many ancient sedimentary rock successions. It appears that significant quantities of dolostone are produced when magnesium-rich waters circulate through limestone and convert calcite to dolomite when some calcium ions are replaced by magnesium ions (a process called dolomitization). However, not all dolostones appear to be formed by such a process, and their origin remains uncertain.