Dolostone (formerly "dolomite") is a chemical sedimentary rock composed of the mineral dolomite (CaMg(CO3)2 - calcium magnesium carbonate). 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 around where I live).
Most dolostones are grayish, microcrystalline-textured, and have vuggy porosity. Vugs are irregularly-sized & shaped cavities. Some dolostones are fossiliferous. Fossiliferous dolostones usually have poorly-preserved fossil "ghosts" (see below). 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 dolomite-forming process is still a significant unsolved problem in sedimentary geology - "the dolomite problem".
Dolostone of Silurian age with vuggy porosity (= the irregularly-sized & shaped holes) and fossil “ghosts” (the somewhat rounded, empty structure on the right side is an external mold of a fossil brachiopod).