"I am formed deep underground due to heat and pressure. I do not melt. Slate and Gneiss are examples of me, while Pumice and Granite are not."

Answer 1

You are describing metamorphic rocks. Metamorphic rocks are formed from existing rocks (either igneous, sedimentary, or other metamorphic rocks) that undergo a transformation due to high heat, high pressure, or chemically active fluids, but they do not melt in the process. Slate and gneiss are indeed examples of metamorphic rocks, while pumice (an igneous rock) and granite (also an igneous rock) are not.