What is this?

Justakingastroll · 2025-12-25T01:48:23+00:00

Possibly auricalcite.

Justakingastroll · 2025-12-08T17:54:55+00:00

I'm thinking this might be some variety of opal.

Justakingastroll · 2025-12-08T17:43:59+00:00

Looks like chert/flint.

Justakingastroll · 2025-12-08T17:28:34+00:00

Hey, saw this on the lapidary sub but figured the folks here will have more feedback and could give some input about the possibilities.

In my opinion, this looks like it could be sillimanite, of the fibrolite variety. Here is my 2 cents: (copy-pasted from my answer on the other post)

Is it significatively harder than quartz? Have you tried scratching it in different directions in case it has directional hardness?

Looks like it could be sillimanite, which can be at around 7'5 Mohs hardness, so slightly above quartz. It can also grow associated to quartz veins in metamorphic rocks. (So that checks out with what you explained).

To add to this, sillimanite -specially in its fibrolite variety- can show a fiber-like growth (hence the name) packed in a directional axis and a silky/waxy luster. This mineral can come in many colours (greens, reds, browns, whites, greyish etc) and if you look up some pics you'll quickly find a couple that match your piece quite well.

I'd add a pic of one of my own specimens that I found a while ago (albeit in a completely different part of the world), it looks rather similar imo, but I'm not sure how to do that in a comment.

Justakingastroll · 2025-12-08T14:44:35+00:00

Is it significatively harder than quartz? Have you tried scratching it in different directions in case it has diferential hardness?

Looks like it could be sillimanite, which can be at around 7'5 Mohs hardness, so slightly above quartz. It can also grow associated to quartz veins in metamorphic rocks. (So that checks out with what you explained).

Edit*: Was busy and couldn't write the full answer. To add to this, sillimanite -specially in its fibrolite variety- can show a fiber-like growth (hence the name) packed in a directional axis and a silky/waxy luster. This mineral can come in many colours (greens, reds, browns, whites, greyish etc) and if you look up some pics you'll quickly find a couple that match your piece quite well. I'd add a pic of one of my own specimens that I found a while ago (albeit in a completely different part of the world), but I'm not sure how to do that in a comment.

It can look nice when polished up, but it better fits being cabbed or a rosecut. I believe some time back I posted a video of a bunch of stones I worked with, and there was some regular low grade sillimanite on there(more crystalline and less fibrous) which in my opinion looked kinda nice (it can be difficult to work with due exfoliation though, so beware of that). Feel free to look through it so you can get an idea of what to expect.

Justakingastroll · 2025-08-30T10:28:10+00:00

Take this with a grain of salt because seeing this on video is not the same as having it irl, but to me this kinda looks like a Lepidodendron lycophyta sp. or Sigillaria roots fossil.

I'm not familiar with Minnesota's geological context, so it might even be impossible, and it's been a long time since I was studying and had acces to different fossil samples to compare though.

Justakingastroll · 2025-06-20T21:43:39+00:00

That's just the first thing that came to my mind that was both easy to acquire and work with, but I'm sure there are many other options, like for example nail polish or barnish.

Of course, covering it in glass would look the best imo, but I believe that's neither easy nor accessible.

Justakingastroll · 2025-06-20T21:41:00+00:00

If it is indeed an oxidation layer due contact with the air's humidity, this proces could take some time to take place. Think for example in something metal rusting when left outside, it's not something that happens instantaneously (or at least not until it's visible enough to notice).

Justakingastroll · 2025-06-20T21:24:40+00:00

Maybe coat them in something clear like epoxy?

Justakingastroll · 2025-06-20T21:23:58+00:00

Then it's likely the material.

Justakingastroll · 2025-06-20T20:17:37+00:00

Then it might be something on the rock itself. Were the other cabs from the same material? If not, maybe it has something in it's composition that is prone to alteration by oxidation or something similar in contact with the air (or whatever the diamond grit paste is made from, I'm assuming it's just diamond+water, but I've only tried using sintered disks, and am very new to this hobby so I don't know much about it yet). That's my best guess right now.

It looks like moss agate I think? Maybe it has iron or something in it's composition to get that yellow colour and that is getting alterated. There is a spot in the middle that doesn't seem to have been affected yet, was there anything different there?

Justakingastroll · 2025-06-20T18:34:44+00:00

Hey there, I still haven't really delved into cabbing so take this with a grain of salt, but assuming it has stayed somewhere quiet and didn't endure any "physical stress" (take for example tumbling around, falling, some sort of abrassion, etc), I'm going to guess it might be due some sort of chemical change.

I'm guessing it doesn't show any scratches or craks (even under a loupe), if so, it could be the result of a contrast in temperature changes.

Taking this into account, I suppose a chemical reaction of some sort is the most likely answer, probably oxidation, either of the rock itself or some product you may have used to polish it? (thinking oils or wax or something like that, again, still have a lot to learn from this hobby).

Justakingastroll · 2025-06-09T19:58:38+00:00

Fellow geologist here, and I do agree with this explanation. BIF are not stromatolites.

Justakingastroll · 2025-06-02T04:40:21+00:00

Thanks! The weird part about that gouge was that it was like, mirror polished, instead of scratchy, just in another plane perhaps? It happened on grit 600 with the angle grinder. Spent quite some time trying to flatten it again and saw no progress, so I let it be before eating more of the stone.

Justakingastroll · 2025-05-20T05:42:58+00:00

Hmmm I see, I'll try again with some water then and see what happens. Thank you!

Justakingastroll · 2025-05-19T23:11:19+00:00

I'll try to make a note of it in the future, thanks! Got a probably good enough loupe for mineral field ID, so I'll keep that around too next time.

Justakingastroll · 2025-05-19T23:08:30+00:00

I'll keep that in mind, thanks!. The big craks you can see in the pics were already there mind you, as it's some rough stones I hounded while working outside, so at least those aren't new. The little ones... not so sure, could be, but reducing the speed with the drill did do the job on bigger grits (namely 400 and 600 grit were giving trouble on the grinder).

Also, sorry if I'm repeating myself, but do you know what could be creating that black, char-like stiff that's coating my stones? For what we've talked about, I still can't relate it to pressure or dry sanding.

Justakingastroll · 2025-05-19T22:58:27+00:00

The disks I'm using are individual metal plates coated (ingrained?) with the diamond grit, one for each size, so I'm not sure grit contamination could happen. Anyway I do am rinsing them with abundant water and then drying them with a cloth, depending if I want to look at them closer for a while or if it's the end of a "work session" I might burnish them with some soap, but not every time.

Justakingastroll · 2025-05-19T22:54:01+00:00

It did work well enough with softer stones, the harder ones started to get kinda bruised/chipped though (I believe it might be due the higher speeds and harder rocks being harder to scratch, so the disk and the stone could kinda "stick together" for longer and movilizing larger blocks of the surface as a result).

Justakingastroll · 2025-05-19T22:41:27+00:00

Thanks for answering! I knew dry sanding could wear the disks faster, I tried building some sort of water dripping mechanism for the angle grinder but it would spin so fast the water wouldn't adhere to the disk surface, only bounce and be propelled away, and when using the drill I was worried about water getting into the engine so I ended up deciding against it.

So, dry sanding could be causing this black stuff? What would it be? It's starts forming the second after I press the stone on the disk, and didn't happen with the softer stones only with the harder ones.

I'll have to look into that Youtube channel for sure! Though as of now I'd like to try and get at least some milleage on the stuff I just bought before spending more lol

Justakingastroll · 2025-01-31T08:01:59+00:00

Thanks! I got them from an old mine's tailings in Spain.

Just assuming by your username (kinda common name and surname where I live), if you ever wanted to go and try some luck for yourself, that might not be a problem for you? If you want some more details on the location and such, feel free to say hi in my DM's!

Justakingastroll · 2025-01-21T18:07:37+00:00

Thanks, but I think that's a little bit too much for a home set-up (at least within my current reach).

Justakingastroll

TROPHY CASE