A crack is a when the crevice goes through the whole thickness of the stone, i.e. it goes through and through. It separates one section of the slab from other like a cut.
It is very tough to repair a crack cause no process can give the marble strength it originally had, it makes it very difficult to use a cracked piece.
A process known as Filling can be used to fix the slab to a certain extent. Filling is the process of filling up the crack with an adhesive and coloring which matches the texture of the stone. This can temporarily fix the slab, if taken proper care the same can be installed on a floor and used without any issues.
A solid marble slab might sometimes crack even after installation. This happens due to improper installation. One of the major reason being using a huge size piece which results in crack development over time.
A cavity is when the crevice does not go through the entire thickness of the stone.
A cavity goes only 0.2mm to 0.5mm in depth which makes it very different from a crack. Cavities are a result of natural stone formation and are present in every natural stone.
Cavity is nothing to be afraid off, it does not cause any negative effects on the stone as it does not compromise any of its characteristics.
With today’s installation techniques it can easily be dealt with. A proper chemical filling and polishing makes the surface smooth and shiny.