GeologyRocks

Not sure the top four are fossils. The fifth has circled the one I don't think I can identify. Sponge??

The last looks like a colonial coral.

Caveat: in my geology degree one could drop palaeo at the end of the first year - I did so for a very good reason - i was rubbish at it!!!

1 and 2 remind me of tufas. They are freshwater limestones that precipitate in rivers and pools. Where they precipitate around grass and reeds, you often get a texture with long tubular holes once the grass has died and decayed. I'm not really confident about that at all however. Did you find it near a stream?

3 looks like some kind of mineral concretion to me, but I wouldn't like to say what kind. 

I found it buried halfway on the mudbank of a river. Its the biggest rock I've got. I'll do some more research about tufas.  

After doing some research I believe it is Tufa. This only leads to more questions. How old is it? Should I break the rock open? Is it worth anything? Thanks for your help. 

The top 2 are definitly calcerous sinter and formed like already explained.

 The 3rd foto from top is a concretion like mentioned, though I cannot decide wether it is an iron/limonite concrection, phosphoritic or something else.

 The 4th I don't know. If this is a fossil it is badly preserved. Though I wouldn't even rule out that we are seeing nothing more than an oddly shaped pebble or rock. I have some that even fooled the palaeontology profs into thinking it's a piece of a tree trunk.

5th. the red encircled piece seems like a fragment of a coral to me. Nevertheless I do recall also sponges with such shapes though they are rather rare and the only similiar ones I know are modern freshwater sponges from Lake Baikal. Sponges are fairly hard to preserve unless they get silified or mineralised very fast after death. I'd place my money on coral fragment.

 The last one is hard. A scale would be useful to see how big it really is. I am guessing on coral here, too.

 Breaking open the tufa? No! It is very unlikely to find anything interesting inside and they usually look much better in one piece. The age could be fairly modern. I have seen sinters form within only a few years. If you found it in the wet mud of a river it could only be a few decades old. I found some here in germany with living moss already incorporated into them. If you want to get very into it you could try some C14 dating of the tufas but I dont think it would be worth the effort.

You could sell all six of them to rock lovers for 50 US cent to 1 dollar each. I would give you 1 or 2 dollars for all 6 at best but thats already too much i think and only a friends price. Those tufas however can look fairly nice as decorations in aquariums or your garden.

Thank you very much, that was a big help.

You're welcome. If you have more rocks idling around post them here and we will all see what we can do about it.

No idea about the first four but the fifth (circled item) could be a bryozoan, and the sixth (depending on scale - tried to estimate from wood grain) looks like large coral polyps possibly a colonial rugosan, or tabulate with such thick septa.

Hey! Welcome to GR!

The top one might be coral. The last one is a coral (sorry, not good at species level ID). Pics 3 & 4 look like concretions of some kind?

The "rod" one is one of: sponge, coral, echinoid spine?

Where are these rocks from? If they were found in a river, I don't think they were found a long way away from the source.

 The first rock is tufa, I done a lot of research on it, and I'm sure thats what it is. The last one is a kind of coral, its one of the best preserved fossils I have. All of these rocks came from Kentucky,US in the Nolin River, where I've always hunted for fossils. I've never had to dig for anything, they are laying all over the place. Some were in the water, others were on the bank of the river. I break open some of the larger rocks to find fossils inside, thats how I found the fifth fossil.
Thank You