Limestone is a sedimentary rock made primarily of calcium carbonate (calcite) or calcium-magnesium double carbonate (dolomite). Tiny fossils, shell pieces, and other fossilized debris is commonly found in it.
On careful observation of the stone surface, these fossils are frequently apparent to the unaided eye, but this is not always the case. The grain of some limestone kinds is exceedingly fine.
Limestone is often grey in colour, but it can also be white, yellow, or brown in appearance. It's a soft rock that scratches readily. Any ordinary acid will cause it to effervesce.
Coquina, a matrix of whole or fragments of seashells loosely bonded by calcite, to oolitic limestones and microcrystalline limestones, whose structures are so fine that they can only be seen under magnification, are examples of limestones that vary widely in texture and porosity.
During important geological events, limestone deposits can undergo metamorphism and recrystallize as marble.