Here's the breakdown:

* Limestone and sandstone are sedimentary rocks. They form from the accumulation and cementation of sediments like shells (limestone) or sand grains (sandstone).

* Gneiss forms when existing rocks (like granite, shale, or even other metamorphic rocks) are subjected to intense heat and pressure deep within the Earth's crust. This process transforms the original rock's minerals and structure, creating a distinct layered or banded appearance.

Think of it this way:

* Sedimentary rocks are like building blocks made from sand, mud, and other debris.

* Metamorphic rocks are like those blocks after they've been baked and squeezed into something new and different.