What’s this?

[3RZUL13]

Pulled this from roughly 3-4 feet deep in my backyard on the far west/nw edge of San Antonio. Found a rock bed as I was digging. Odd because until then the soil was actually soil…it was dark and was easy to shovel. I’ve dug holes all around my property and at many other spots around this area and I’ve always hit a very solid layer of limestone before even getting a foot deep. So 3 feet was noteworthy. And that was about 8 months ago. It took me that amount of time to uncover this one. It’s a hair shy of  2lbs4oz/ just over 1kg. 
 

Any thoughts on it?

[Ludwigia]

Another interestingly shaped rock. Could be conglomerate or even manmade mortar or concrete.

[digit]

If you say you are hitting a limestone layer in your backyard, this appears to be an isolated piece of limestone that may have been found deeper possibly because it was down in a solution hole. I had solution holes (often filled with debris) on my property in South Florida.

 

Information on solution holes:

 

https://waterymiami.medium.com/solution-holes-in-the-florida-everglades-ccfd818559b7

 

 

https://jitneybooks.com/everglades-are-full-of-holes-with-acid-eating-the-miami-limestone/

 

 

Cheers.

 

-Ken

[Brandy Cole]

It looks to be a pretty standard chunk of Texas Hill Country limestone.  I don't see any sign of fossil material.  Limestone is one of those notorious fossil fakers.  

 

It often breaks off or weathers into suggestive shapes.  Also, its texture often contains small holes that can roughly resemble bone texture. 

[JohnJ]

In your area are many "karst limestone" geologic features.  Pieces of those features (rocks) often have strange textures, divots, and holes formed be the percolation of water through the Edwards Aquifer.  

 

Check out you local geology here.

[3RZUL13]

Forgive me if this is an ignorant question. But am I incorrect in understanding that limestone will react with vinegar? On account of the calcium carbonate? I’m only asking because all of the pieces I’ve posted have been soaked in vinegar. Sometimes varying amounts of time but in spite of it being a foolish approach in regard to fully preserving the finer attributes of the piece…but I was more so this entire time just trying to determine whether there was something there besides limestone in the first place at all—so i definitely was in the mindset of remove the limestone so all pictures I’ve posted thus far have been soaked in vinegar and scrubbed and rinsed. Some more than once even lol. I feel you guys on the limestone. And like I said please do forgive my ignorance if I’m incorrect in my understanding of the process, and I’d be grateful if you felt inclined to explain it for me so I don’t continue to be ignorant on the subject. But I was of the mind that this was more or less what was beneath the limestone layer, once scrubbed and allowed to sit for over a week and it’s at the point that crystals have now  formed a thin layer all over.  
That all being said, does your impression differ at all? It’s perfectly okay if it doesn’t I just now am concerned that I’ve been looking at this all wrong from the jump lol. No matter what I appreciate y’all!

[Bone Daddy]

Second what was said above. This looks like carbonate karst limestone. It can be fossiliferous, depending on the locality. I find weird shaped chunks of it here in Florida.

 

[digit]

1 hour ago, 3RZUL13 said:

Forgive me if this is an ignorant question. But am I incorrect in understanding that limestone will react with vinegar?

In general, that is true. Calcium carbonate comes in 2 forms: aragonite (limestone) and calcite. The calcitic form is much more resistant to acids like vinegar. This is often used to an advantage to clean aragonitic limestone encrustations from (some) mollusk shells and echinoid tests that are made of the harder calcite.

 

https://en.wikipedia.org/wiki/Calcium_carbonate#Structure

 

Often limestone that is buried in the presence of silica sand will transform over time into microcrystalline quartz (chalcedony aka chert). This makes it much more resistant to acids. It also makes it much harder, more brittle, and capable of taking an edge which is why chert was used by the Native Americans to fashion stone tools and projectile points with a lasting sharp edge.

 

These may be possible explanations for why your specimen does not visibly react with weak acids like acetic (vinegar). Likely, a stronger acid like muriatic (hydrochloric acid) would more actively react with your piece to dissolve it while fizzing and producing CO₂.

 

 

Cheers.

 

-Ken

[Brandy Cole]

@digit. I had been wondering about the hardness difference because the limestone in the Texas Hill country seems to have a  different character than the limestone I would typically find in waterways in the southeast part of the state.

[Harry Pristis]

Consider potential differences between caliche and ancient sea bottom.