Snowflake obsidian with ???

[Maine USA Beachcomber]

Hi,
Found this on the beach after a storm yesterday mixed in with thousands of other rocks. Pretty certain it's snowflake obsidian. Can anyone explain to me what the brownish "inside" material is? It almost looks to me like the obsidian is a cortex around the snowflake material. Is this common?

Thanks for any insight.

 

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[Tigger]

Pretty sure it's not obsidian. The fracture on the first pic, bottom bit, doesn't look right. It reminds me more of our agatized coral here in Florida. Take a good magnifying glass to the white bits. You can also drop vinegar on each of the different parts to see if any fizz. It's a really neat one though!!

 

[Maine USA Beachcomber]

Oh wow thanks. Never thought it could be coral in Maine but been finding some other Carribean kinds as well. Is that the name of the coral? I'd like to reasearch it.

I looked w my 10x lens to the white and brown areas and they looked chalky to me. No pattern that I could make out.

 

[Tigger]

Try some vinegar on them - just a drop or two. It will probably fizz. I don't think your rock has been agatized - looks more like chert to me? but we'll let the experts chime in - I just thought I'd get you pointed in another direction.
The "Acid Test" for Carbonate Minerals and Carbonate Rocks

 

[Maine USA Beachcomber]

Try some vinegar on them - just a drop or two. It will probably fizz. I don't think your rock has been agatized - looks more like chert to me? but we'll let the experts chime in - I just thought I'd get you pointed in another direction.
The "Acid Test" for Carbonate Minerals and Carbonate Rocks

Used the vinegar, definitely dissolved the white a bit.

 

[Eu_citzen]

I was going to guess flint, but chalcedony is just as good of a guess. The white is made up of calcite - which reacts with vinegar as you noticed.

 

[Maine USA Beachcomber]

I was going to guess flint, but chalcedony is just as good of a guess. The white is made up of calcite - which reacts with vinegar as you noticed.

Is it odd for the calcite to remain in the chert for extended ocean voyages lol? Maine has no natural chert deposits that I know of. This also looks like a black matrix of chert if that's what it is. The stuff inside isn't cherty. It's chalky. Does a chert matrix ever cover chalky substances? Not questioning you, just trying to learn about chert. I like it a lot.

 

[Tigger]

Is it odd for the calcite to remain in the chert for extended ocean voyages lol? Maine has no natural chert deposits that I know of. This also looks like a black matrix of chert if that's what it is. The stuff inside isn't cherty. It's chalky. Does a chert matrix ever cover chalky substances? Not questioning you, just trying to learn about chert. I like it a lot.

The holes that you're seeing in the chert, represented by either an actual hole, or the circle of calcite/limestone/whatever, are signs of the branches of coral or other fossils that used to be there. The black (I was guessing chert) formed around them, then the fossils eroded out of it, and the calcite/whatever filled those coral-shaped voids, some all the way, sometimes just coating the voids. Our state rock is agatized coral, which I find dredged from the Gulf. I don't know how far a rock that size could travel, but I'm guessing it washed up from the ocean, so you may not be familiar with it.

I know I say this a lot in my posts, but it is really helpful to touch base with your local natural history museum, college or rock/gem club. It's far easier to really id these things when you're a local and you have it in your hands. But the big win for you is that they know about other local cool rock locations, maybe have fieldtrips, fun stuff like that. I was in a quarry yesterday with our club, having lots of fun finding fossils and chert and calcite and talking rocks with new folks. I would really encourage you to look into your local club, and see what they have to offer!

 

[Eu_citzen]

Tigger nailed it. The chalky white calcite has to do with the formation of the rock, which obviously is underwater. And water can have minerals dissolved, which when dropped out of solution, can fill voids and the likes. That's how I'd imagine it.

 

[Maine USA Beachcomber]

The holes that you're seeing in the chert, represented by either an actual hole, or the circle of calcite/limestone/whatever, are signs of the branches of coral or other fossils that used to be there. The black (I was guessing chert) formed around them, then the fossils eroded out of it, and the calcite/whatever filled those coral-shaped voids, some all the way, sometimes just coating the voids. Our state rock is agatized coral, which I find dredged from the Gulf. I don't know how far a rock that size could travel, but I'm guessing it washed up from the ocean, so you may not be familiar with it.

I know I say this a lot in my posts, but it is really helpful to touch base with your local natural history museum, college or rock/gem club. It's far easier to really id these things when you're a local and you have it in your hands. But the big win for you is that they know about other local cool rock locations, maybe have fieldtrips, fun stuff like that. I was in a quarry yesterday with our club, having lots of fun finding fossils and chert and calcite and talking rocks with new folks. I would really encourage you to look into your local club, and see what they have to offer!

Great advice thanks. I did speak to a Mainah who said it could also be basalt, which is common to Maine. But the same mechanism, where only bubbles instead of fossils. I think he said the term was "Amygdaloidal." Don't know how to determine which it is. I need to research the basalt/chert properties & compare.
I think you got me headed on the right track for sure.