Here's why:

* Igneous rocks are formed from the cooling and solidification of magma or lava. Sapphires are not formed directly from molten rock.

* Sedimentary rocks are formed from the accumulation and cementation of sediments. Sapphires are not formed from these processes.

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemically active fluids. Sapphires are formed when the mineral corundum (which is the main component of sapphires) is subjected to these metamorphic conditions.

The process: Sapphires typically form within metamorphic rocks called marble and gneiss. These rocks contain corundum, which gets recrystallized and often takes on a beautiful blue color due to trace elements like iron and titanium.