How can I make it so that my marble floor reflects what's above it/sitting on it? I would look at some of the material files from HL2, but, I can't really think of any examples of this in the game.
This is just an example and you can change the values for $envmaptint for your likes. Then you add a env_cubemap into your map and compile it. After that you have to load your map and type buildcubemaps into your console. And voila there you go.
Edit: I think there are no marble textures in hl2 you have to make your own or download some.
There are some nice marble textures packed into djbourgeese's (I know I spelt that wrong) cs_xcom map. I used bspzip to pull them out. If you are hopeless with console applications, I can just email you the textures, because I am just a nice guy