Limestone itself doesn't have a single crystal structure. It's a sedimentary rock, which means it's formed from the accumulation of sediments like mineral fragments, shells, and other organic materials.

The primary mineral component of limestone is calcite (CaCO3), which has a rhombohedral crystal system. This means that its crystal structure is defined by three axes of equal length that intersect at angles that are not 90 degrees.

However, limestone often contains other minerals, including:

* Aragonite (CaCO3), which has an orthorhombic crystal system.

* Dolomite (CaMg(CO3)2), which has a rhombohedral crystal system but with a slightly different arrangement of atoms than calcite.

The specific crystal structure of the calcite in limestone can vary depending on the formation process and the presence of other minerals. For example, some limestones are microcrystalline, meaning the calcite crystals are very small. Others are macrocrystalline, meaning the calcite crystals are larger and more easily visible.

So, while calcite, the main component of limestone, has a rhombohedral crystal structure, limestone itself is a polycrystalline rock composed of various mineral components with different crystal structures, resulting in a complex and varied overall structure.