Here's why:

* Hard: Dolostone is known for its hardness, often exceeding that of limestone.

* Forms inside limestone: Dolostone forms through a process called dolomitization. This is where magnesium-rich fluids react with calcium carbonate (limestone) to replace some of the calcium with magnesium, forming the mineral dolomite. This process often happens after the initial limestone formation, essentially transforming it into dolostone.

Let me know if you'd like to learn more about dolostone!