File:Fossiliferous dolostone 1.jpg

English: Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Dolostone (formerly “dolomite”) 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 problems in sedimentary geology - “the dolomite problem”.

The specimen seen here has some fossils preserved as external molds. The coiled structure at center-right is a fossil gastropod / snail.