Agatized coral fossil

[Sherry.1964]

Found this in a little creek that runs behind where I live in east Tn. I need help in identifying it. It weighs about 7lbs. 

 

[Shamalama]

Can you post some larger, closer pics of the specimen? right now I only see a large chunk of Chert or quartz. I do not see any diagnostic structures that would indicate coral.

[digit]

First, welcome to the forum!

 

Agreed. I'm seeing what looks like a nice piece of chert (microcrystaline quartz) but I'm not seeing anything that would indicate coral. If anything, it looks more similar to petrified wood.

 

We find silicified fossil coral just north of Florida in southern Georgia in the Withlacoochee River. It dates from a time in the Eocene when the peninsula of Florida was cut-off from the mainland by a shallow waterway that allowed the Gulf Stream to bypass Florida through what was called the Suwannee Channel. The area that is now the Withlacoochee River was a reef with corals. There were probably a diversity of coral but oddly (probably due to a quirk of preservation) only a single species seems to be present (in great quantities) in fossilized form today--Siderastrea silicensis. The section called FLORIDA SUBMERGED in this paper on Florida geology has a drawing which demonstrates what the area was like then:

 

https://www.stetson.edu/other/gillespie-museum/media/Florida Formations EXHIBIT TEXT-FULL for website.pdf

 

In addition to fossilized chunks of this large boulder coral, the area has a lot of silica-rich sand. Over time rain has dissolved the silica and replaced the calcium carbonate of the coral skeleton. In some cases the chunk of coral is completely silicified turning it to what is known as calcedony (chert) which was used by the Native American peoples of the area to make projectile points, knives and scrapers. Flakes from the production of these artifacts can be found on the land around the river. Occasionally, the calcium carbonate of the coral skeleton would be dissolved away leaving a void. Layers of silica-rich water would build up on the inside surface of the hollowed coral leaving a "geode" of sorts (though the vesicles are not of volcanic origin). When the interior of one of these corals contains concentric bands of different colored silica material it may then be called an agate. These hollow corals may exhibit rounded globes of quartz that have built-up from points inside forming what is known as botryoidal texture since it resembles a bunch of grapes (botrys in Greek) or it may be covered with tiny quartz crystals that make it look like it is dusted with sugar in a form called druzy. Quite often there is a milky white layer called common opal which, as indicated from the name, does not show the colors or have the value of gem quality opal. Here is a trip report from one of our members @Sacha who often hunts the Withlacoochee and finds some fabulous pieces.

 

http://www.thefossilforum.com/index.php?/topic/74351-may-15-16-17-hunt-for-coral/

 

You'll notice from looking at his pieces in this post that they often show the coral polyp pattern on the outer surface of the coral or show the linear striations of the coral polyp as it was growing and extending. I'm not seeing any indications of such a pattern on your piece and that is something we would expect if it was a silicified piece of coral. To be agatized (a term commonly overused for silicified) it would also need to show banding.

 

While it doesn't seem to be a piece of agatized coral it is a very cool looking rock. It might be worth searching the internet to see if you have any rock/mineral clubs in your area or lapidary clubs which might have rock saws and polishing equipment to open this rock and better see what's inside. If you have access to someone with a tile saw or better yet a saw for cutting pavers you might be able to get this cut and see what it looks like inside.

 

Was this the only rock that looked like this in your creek? I'd go back and see if you can find some similar rocks to this. Smaller pieces of this might look nice if run through a rock tumbler.

 

 

Cheers.

 

-Ken

 

[Sacha]

The cleavage fractures look more like feldspar to me.