File:Dolomite-cemented dolostone breccia 1.jpg

English: (crack surface)

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Dolostone is a chemical sedimentary rock composed of the mineral dolomite (CaMg(CO3)2 - calcium magnesium carbonate). Like limestone and calcite, dolostone and dolomite will readily bubble in acid, but only when powdered. Dolostone can be jokingly described as the “ultimate non-descript rock”. It often looks like driveway gravel. Well, there’s a reason for this - a lot of driveway gravel is dolostone (at least in the part of the world where I live).

Many dolostones are grayish, microcrystalline-textured, and have vuggy porosity. Vugs are irregularly-sized and irregularly-shaped cavities. Some dolostones are fossiliferous. Fossiliferous dolostones usually have poorly-preserved fossil “ghosts”. Rarely, fine-grained dolostones have soft-bodied fossil preservation.

Crystalline-textured dolostones appear secondary in origin. They are typically interpreted as chemically-altered fossiliferous limestones. Some dolostones look primary, but how they formed is not entirely clear. Chemically, all that's needed to form dolostone is the addition of magnesium (Mg) to limestone. The details of this chemical change are not fully understood. A few localities on Earth do have dolomite or protodolomite forming now, but the detailed story of the dolomite-forming process is still a significant unsolved problem in sedimentary geology - “the dolomite problem”.

Seen here is a dolostone breccia - the dark-colored, angular pieces are thinly-laminated dolostone clasts. The dolostone fragments are cemented together with pale pinkish-colored, coarsely-crystalline dolomite cement.

This specimen has no provenance information, but it's probably from midwestern America. It may be a fault breccia.