What is this rock?

Our foremost rock expert won't be here until July 4 or bit later. Patience..

As erstwhile rockhoudd, I see a real challenge. Looks like it must be based on quartz

We have some really good rock guys tho Conan...be patient...

Might be some type of brown obsidian. Probably can't tell just from a photo. Will need some tests.

Obsidian is classified as glass, but this looks and feels nothing like glass. I own couple of obsidian rocks and this is something completely different from them.

Betcha Con, it's volcanic tho

Can't you provide a better picture?

looks a lot like "marble onyx". See the layers and the cleavage patterns? its not real onyx, but a type thats seen in aragonite or calcite

drop some vinegar on it to make a little puddle. Then see if it bubbles. If it does, its a marble onyx, if not, its regular onyx("crypto-quartz" based)

Thank you for all the replies! I placed a piece from the rock in vinegar and it indeed bubbled.

marble onyx, in batch deposits extensive enough, is often used for decorative sculpture in the mexican tourist trade. AND, its often use for soft "marble top" counters for butcher shops qnd kitchens.
Its more easily cleaned an kept sterile than most other, harder stones where microscopic cracks although extremely hard, can hide pathogenic bacteria. Softer stone like marble onyx can be cleaned easily and polished with surface sterilizing liquids.

I don't know the basics of rocks as I once did, but it looks like CHOCOLATE/caramel. Yum!

Hi all,

I agree that's an aragonite precipitate. It is likely related to some kind of karstic process, they are really common in limestone caves. Do you know any caves around there?
Plus, the base you show on the second picture seems to indicate it has been detached from the rock where it precipitated upon.
Cheers!

yeh good point about a karstic association.