The very hard sedimentary rock that often forms inside limestone is chert.

Here's why:

* Formation: Chert is formed from the accumulation of microscopic silica skeletons of marine organisms like diatoms and radiolarians. This silica can accumulate within the pores and spaces of limestone, eventually crystallizing and forming the hard, dense rock.

* Hardness: Chert is known for its extreme hardness, often exceeding that of steel. This makes it a valuable material for tools and weapons in prehistoric times.

* Appearance: Chert can come in a variety of colors, often with a glassy or waxy luster. It may appear banded or have a nodular form.

While other hard rocks can exist within limestone, chert is a common and distinctive example due to its specific formation and properties.