Can Anyone Tell Me What This Thing Is?

[Tennguy]

Hi Guys. ORARI here. Like all archaeologists, assorted citizens bring me an "artifact" to identify every once in a while. A nice lady who lives in Georgia sent me the attached photograph of a rock her sister found in Dickson, Tennessee. Dickson is over near the western edge of Middle Tennessee. She wants to know if it is an American Indian artifact. This somewhat fuzzy picture is the best she can do right now. Although she did not say so, she may think this is an artifact because it appears to have a black eye on it with a semicircular eyebrow. After looking at it for a couple of seconds, I concluded to my own satisfaction that this is a naturally formed rock (probably limestone). The smoothness and curved edges suggest to me that this rock was once exposed to a great deal of moving water. The so-called "eye and eyebrow" is some sort of black mineral deposit that tends to accumulate on rocks in raised up concentric rings---in this case with one ring either partially formed or partially destroyed so it looks like an eyebrow. Can you confirm my identification as just a rock? Does anyone seriously think this might be a prehistoric artifact?

I know you guys have seen more rocks that are NOT artifacts than just about anyone. Have you ever seen anything like this, and does anyone know what this thing is in geological and mineralogical terms? I would like to tell her a little bit more than: "This is just a rock." I have seen things kind of like this before, but I cannot recall what they are. Any help you could provide would be most appreciated. THANK YOU!!!

[painshill]

Hi Orari

Your description of the geologic processes that can create these kinds of features is, I think, entirely accurate. They’re known as “orbicles” and when a rock is predominantly composed of minerals in that form, we call it “orbicular”.

It’s very common in igneous rocks and those composed of hydrothermally deposited minerals but also commonly occurs in sandstones where the usual culprit is orbicular calcite. Impure calcite is frequently dark grey but doesn’t usually get as far as black unless co-deposited with iron minerals which also commonly form orbicles in host rocks (notably hematite and magnetite).

Here’s a couple of pictures of orbicular calcite deposits in sandstone from the CSMS Geology Post blog: