Earth, the element in the North Quarter of the sacred Circle. Green is the color. Symbols for an altar to the Earth would include salt, sand, or soil, stones crystals, etc. I've even seen one that included a small yard gnome. ;)
Gnomes are the Spirits that inhabit the Realm of the earth. When I journey to this realm they often show themselves to me and the closets thing I can think of to paint a picture of what they look like (to me anyhow) is a tiny version of the Rock Biter from Neverending Story. About a foot tall they are and they have grey or brown rocky-looking skin. Some of them look like tiny trees with leaves and such for hair. They have dirt under their nails and are always barefoot.
The Earth is where our roots are. It is the foundation stones if you will of life. It has a unique vibration that "hums" to me whenever I'm out in nature.
Meditation:
Lie on the ground (do this outdoors if you can but if not have a plant or dish of soil near you where you can rest your power hand in the soil. Go through your relaxation breathing. As you count backwards from 10 you are getting more and more relaxed. Your breaths are slower. As you sink into this relaxation, think of yourself at the foot of the World Tree and you are slowly going down the roots into an underground cavern. It is a wonderous place. Damp and cool but not uncomfortable. Your voice echoes here. If you sit and wait, you will soon see a Gnome appear seemingly from the rock around you. Greet him/her with great respect. They may look childlike but they are spirits with fathomless knowledge.
Ask him questions if you wish. When you are ready, bid farewell to the Gnome count back up to 10 and climb the roots of the World Tree. Give yourself a few minutes to come back to this plane of existance. Journal your experience if you like. :) I'll journal mine by Monday. :)