This was the last of the four base element tools that I found. I have seen some beautiful carved and burnt wooden pentacles that would make lovely earth tools. Mine is a simple silver-tone coaster. It doesn't have a pentacle on it at all, but it is flat and round. I use it mainly as a blessing spot, sort of a small energy nexus on my altar. If I am making offerings or putting energy into something, it goes on my pentacle. I guess maybe I should call it a shield.
The first time I read about using a shield for an earth tool I thought it was a bit fantasy roleplaying....not that I have a problem with that, it just seemed a bit contrived to me. But the more I thought about it, the pentacle is a shield, just not the kind that stops swords (unless they are magic swords!). The pentacle symbol and tool are often used interchangeably (though most of my pentacle jewelry is blessed towards five elemental balance not just earth). It is very commonly used as a symbol of protection or warding, much like the magic circle actually.
I was going to talk about the pentacle symbol and my thoughts on it's meaning in another post, but it kind of fits here, so here we go. If you think about the pentacle shape, you have the pentagon in the middle, surrounded by the 5 triangles pointing out and finally enclosed in a circle. I see energy as moving through the pentacle in mainly two ways, in or out. The star becomes a focus for either sending energy out along the points from the middle or drawing energy in to the center. Either can be used for shielding or charging, depending on how you want to work things. If you envision the pentacle as a shield, a 'hit' could be drawn inward and absorbed (neutralized) or spread outward and dissipated. For blessing or spellwork, you can draw energy in through the points to charge the item in the center or draw energy in through the center (and the item) and then send it out along the pentacle points to do it's work.
When I was writing about water tools, I almost included my mortar and pestle, because a lot of times they are associated with water. I know people who use them as they would a cauldron. I don't really. Mine is an earth tool. The process of grinding something down to me is a very earth process. But also, I rarely use it for liquid processes, normally it is dry grinding and mixing only.
I know a lot of people use crystals as earth symbols, and I do sometimes. I have a small collection I inherited from another witch as well as rocks I have collected over the years (some from as long ago as grade school). Some I use as symbols of other elements. I have favorites I use to cast circles with (not just as quarter markers, I actually normally use 12 to mark a circle). I'm not sure I would consider my stones tools of earth though, because I do use them for such a variety of things.