FMF - Friends of Minerals Forum, discussion and message board

[ChalaratLaw]

Hi, I found this rock on the beach in Crescent City, northern California while looking for agates around September 2019. I was trying to search for more information about it, and some of the people I asked said it might be fossil, but I can't find anything that looks like it yet. I know nothing about mineral specimens, so if you could please kindly give me more info about it would be very appreciated.

[SteveB]

Chalcedony not fossil. Small red one is chalcedony (simple agate) black and white one exhibits same formation process but more likely a calcium limestone mineral.

[Bob Harman]

I have my doubts that the black and white example is microcrystalline quartz, variety chalcedony. The surface texture and pattern is unlike any chalcedony that I am familiar with.
The surface texture simply doesn't look like microcrystalline quartz.
I would not be surprised if the example is nothing more than a water worn pebble of slag.
Maybe other responders will give their opinion.
Bob

[Bob Carnein]

The red rock appears to be an agate; I agree with Bob that the black one looks like slag. Is it harder than a steel knife blade?

[David Mustart]

The black and white specimen may be slag but I doubt it.
As previously mentioned, try scratching the white and the black with the point of a knife.
Does that test result in a groove in the white?
Does it produce a groove in the black?
Although I doubt it's magnetic, you might check with a magnet. Is it attracted to a magnet?
Also as mentioned previously, it may contain calcite, so the most important test is to obtain a dropper bottle with dilute (10%) hydrochloric acid and put a drop on the white.
Does it bubble?
Report the results of these tests and maybe we'll be closer to an identification.
If you can't make these tests, call any local university geology department, and I expect someone will be glad to help.
Or as a last resort, email me at SF State Univ.:

mustart@sfsu.edu

Whatever it is, the texture is very interesting!

[ChalaratLaw]

Hi,
after I read your suggestions
- I tested it with magnet .. there was no response to magnet at all.
- I tried scratching it with the point of the knife, the black part can't be scratched, the white part came out like a powder but left no scratch mark on it when I rubbed the powder off.
This all I can test for now, I have no idea where or how I can get the hydrochloric acid yet.
Please let me know if you have any more suggestions.
Thank you

[David Mustart]

To advise you on where to get acid or more help with identification, it would be useful to know where you are located.

[Bob Harman]

Any home improvement or hardware store in Northern California will have muriatic acid which is 28% HCl and used for cleaning lime from swimming pools, but simple household vinegar (5% acetic acid) will be fine as your example is very small. The vinegar is safe to use, just takes a few minutes longer to dissolve any carbonates in your small specimen.

But that, for me, is not the question. The real question is that the black area doesn't look like chalcedony (any microcrystalline quartz); so, if also not slag, what might it be? Bob

[ChalaratLaw]

Crescent City, California. 30 minutes away from Oregon border.

[Bob Carnein]

How about obsidian? Are you near Mt. Shasta?

[Birdbrain59]

I think obsidian is a good call. I live in Southern Oregon. We’re kinda known for obsidian

[ChalaratLaw]

So I tried soaking it in the vinegar for 5 minutes, look like nothing changed in the Black part, the white part look like when it wet. I tried again but left it soak for 10 minutes and nothing different.

[ChalaratLaw]

We're around 4 hrs. away from Mt. Shasta my husband has a few obsidian we compared but it's nothing like it, from the feeling when touch or sound when knock it on another rock, this sample more like a rock than glass.

[Bob Harman]

I really do not like repeatedly responding to these type postings, simply because I find giving an ID to small specimens like you have, just from a photo, very difficult at best.
But I will summarize my thoughts and then give you my advice.

I don't think your example is microcrystalline quartz, variety chalcedony. The black surface appearance just doesn't look anything like that to me.
I don't think the vinegar treatment helped with the ID other than to confirm that the black area is not a carbonate.
If the example is obsidian, it should be easy to confirm just by looking at the company your example has: if there is similar obsidian in your area = a good chance your example might be,
but no obsidian or very different obsidian in your area = very unlikely that your example is obsidian.
A water worn slag pebble, to me, still seems a likely possibility.

So after all that, I suggest taking your example to a local club, museum, or university geology department and letting other folks look at it in person. Hope my advice helps, BOB

[David Mustart]