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.