* Sandstone is a sedimentary rock: It forms from the accumulation and cementation of sand grains.

* Gneiss is a metamorphic rock: It forms when existing rocks (like sandstone, shale, or granite) are subjected to intense heat and pressure deep within the Earth's crust.

Here's a simplified explanation of the process:

1. Sandstone formation: Sand grains are deposited, compacted, and cemented together to form sandstone.

2. Metamorphic transformation: The sandstone is buried deep underground, where it encounters extreme heat and pressure. This causes the sandstone to recrystallize and change its mineral composition.

3. Gneiss formation: Over a long period, the heat and pressure cause the minerals in the sandstone to align themselves in bands. These bands give gneiss its characteristic layered or banded appearance.

Key point: Sandstone must first undergo a significant metamorphic process to transform into gneiss. This process involves extreme conditions and takes millions of years.