If I keep my bare foot on a rock under my desk, that doesn't have contact with the earth, am I still grounding because it's a rock from the earth? I have pink marble on my land and want to use it to ground myself with

It's possible to ground yourself wherever you are, using a variety of different techniques. What's important is how it feels for you. In Art Therapy we quite often use earth based materials and the mere process of holding them in your hand can be very grounding. If you would like to use pink marble, I would encourage you to do so. Having your feet planted in a comfortable way, taking deep breaths in and clearing your mind, feeling what connects you with the earth- that's what's most important.