Here's why:

* Sandstone is a sedimentary rock composed of sand-sized grains. These grains are primarily made of quartz, but can also include other minerals like feldspar.

* Quartzite is a metamorphic rock formed when sandstone is subjected to intense heat and pressure. This process causes the quartz grains to recrystallize and become tightly interlocked, creating a very hard and durable rock.

So, sandstone undergoes metamorphism to transform into quartzite.