Here's why:

* Formation: They are formed by the precipitation of minerals from water solutions, usually calcium carbonate (CaCO3). This process is called mineral deposition.

* Igneous vs. Sedimentary: Igneous rocks form from the cooling and solidification of molten rock (magma or lava). Sedimentary rocks form from the accumulation and cementation of sediments, which are fragments of pre-existing rocks, minerals, or organic materials.

Since stalactites and stalagmites form from the deposition of minerals in caves, they fall under the category of sedimentary formations.