The Basic Wiccan Altar

Psywitch, remember I said that you "can" use the pentacle on wood or metal to represent Earth. In traditional Wicca, and even in the Tarot the pentacle does represent Earth. But you may use whatever speaks to you of Earth on your altar if you aren't practicing within a specific Tradition that says otherwise.

There are a couple of other symbols used to represent Earth in other Traditions, but not so much in Wicca. Here's a couple that could be used instead of the Pentacle.

https://www.thoughtco.com/elemental-symbols-4122788

https://en.wikipedia.org/wiki/Earth_symbol