Any ID on this?

Can you give us any more info on it, like where it was found and under what conditions etc?

Yeh sorry,

I didn't find it. The man who posted found it in madagascar, saying that it was pulled out of a river alongside a shark fossil, claiming it was the fin. We don't think it can be the fin as they don't fossilise.. so we would be good to see what you guys think. Sorry can't give much more info other than it failed the Calcite test.
Possible Stromatolite?

2 other tests you may wish to try. (It may not be a calcite but aragonite-)
So you must take a little piece and smsh it up and then add the acid.

Without doing any detailed min testing, try msshing up a small piece and spray it with ninhydrin you(can get that from a biology supply house) let me know what color shows up.
Otherwise Id suggest someone do a THIN SECTION and a microcopy under polarized light.

It sure looks like a carbonate.

PS, I didnt add a hardness test. It also could be a type of onyx(which is a softer quartz , like chlcedony that forms layers)

See if you cant find any info re: ONYX from Mdqgascar. I hqve no ideaa whether there is ny but it could be something as simple ass as that.

I'd guess it's chalcedony, don't know in what form

onyx is a chalcedony

Thanks Man, been a while

It's funny that you mentioned "shark" because my first impression was that it reminded me of a shark's tooth in overall shape. It doesn't however have any of the standard surface features I associate with sharks teeth. There is nothing in the photo to reference the size. Is this thing very large or small?

Sorry for some reason my post didn't show up..

Thanks very much for the help.
I will suggest the tests and I hadn't thought of onyx at all but it most definitely could be. Another option is tabulate coral.

Size
Length: 22 cm
Width: 20 cm

I see biological features in the first photo. The second photo reminds me of oyster layers.