Here's why:

* Igneous rocks form from the cooling and solidification of molten rock (magma or lava). Beryl crystals are often found in igneous rocks like granite and pegmatite.

* Sedimentary rocks form from the accumulation and cementation of sediments. Beryl can be found in sedimentary rocks, but it's not formed within those rocks. It's usually eroded from igneous sources.

* Metamorphic rocks form when existing rocks are transformed by heat and pressure. While beryl can be found in metamorphic rocks, it's not directly formed through metamorphic processes.

So, while beryl can be found in various rock types, it's primarly an igneous mineral that crystallizes within cooling magma or lava.