What is this rock?
January 13, 2005 11:08 AM   Subscribe

Can anyone identify what type of material this rock is made of? Its blue and purple and sort of sparkles, but that stuff is on a black material. Is it an odd piece of coal? Airplane goo? Dirty salt? I found it sitting in the middle of a parking lot near Boston... which seems like an odd place to find such a thing.
posted by jwells to Science & Nature (14 answers total)
Is it heavy? Does it scratch easily? It looks kind of like the rough granite I've seen piled up on either side of train tracks, but without being able to touch it I can only guess.
posted by cmyk at 11:15 AM on January 13, 2005

Response by poster: The scan makes the size look much larger than it is... it's about an inch by a half inch, so there isn't much weight at all. The scan also makes it look a bit different than in natural light, as it seems to be composed of thousands of little blue/purple crystals all over it, so there isn't really a surface for me to scratch. I can't dislodge any of the little crystals with my thumb though. The scan highlighted a few brownish spots on it that could be rust. Maybe that'll help?
posted by jwells at 11:32 AM on January 13, 2005

Hmmm. Try this: Rock Key.
posted by cmyk at 11:38 AM on January 13, 2005

At a guess, it's a quartz or feldspar with pyrite intrusions (the yellowish/gold flecks), most likely a granite. How hard is it?
posted by bonehead at 11:44 AM on January 13, 2005

Sure looks like granite to me.
posted by Specklet at 11:53 AM on January 13, 2005

My geology prof used to tell a joke over and over: "If a rock is being gneiss, don't take it for granite." (ducks)
posted by goatdog at 12:08 PM on January 13, 2005

Response by poster: Darn, I'd been to that Rock Key page but didn't scroll down far enough to find the troubleshooter. Let me make sure I have my answers to it right. I said it has crystals (looks a bit like sugar really, just with purple and blue instead of just white), no layers, light gray, scratches glass, and is medium grained... all of which produces an answer of granite. Which is what your all saying :-) The only one I'm not sure of is the medium/fine grain question. If I answer fine it's rhyolite. How do I differentiate between the two?

I thought granite looked more like gray/white marble than like this. Is that something else?

Thanks for helping out everyone... I had no idea rocks could be this interesting.
posted by jwells at 12:14 PM on January 13, 2005

It looks like a lot of JPEG artifacts to me.
posted by grouse at 12:17 PM on January 13, 2005

The sparkly stuff looks like mica to me. I see lots of rocks that look like that in the Philly area, but i have no real knowledge on the matter, besides reading a few John McPhee books.
posted by 31d1 at 12:40 PM on January 13, 2005

Looks like a lot of granite I used to find in New Hampshire. Granite is quartz, feldspar, and mica all mixed up, and the proportions can differ. This looks like it's really heavy on the mica.
posted by rustcellar at 1:19 PM on January 13, 2005


My Ex-Fiancee, who was a geology major, used to tell that joke to me all the time. Thanks for the grin. I know wayyy more than I should about rocks. I also say granite.

"If a rock is being gneiss, don't take it for granite."

posted by Nenna at 1:36 PM on January 13, 2005

Grain size is referring to the "texture" of the rock (very nice intro explanation with examples here). A rhyolite has aphanitic (fine) texture while a granite has a phaneritic texture.

To convince you that what you have is probably some form of granite, here's a nice photo of a typical rhyolite, versus some nice granites

The reason rocks have different crystal sizes is directly related to how long they took to cool (for instance obsidian is formed almost instantly, and has a glassy texture: no grains whatsoever), you can tell a lot about the environment under which a rock formed from the grain as well as from the constituent minerals, since some are highly indicative of a particular set of geologic conditions. I think rocks are great! :)
posted by nelleish at 2:10 PM on January 13, 2005

Another vote for granite. There's a lot of it in that area - neighboring NH is called the "Granite State."
posted by sophie at 3:20 PM on January 13, 2005

Response by poster: Hi everyone. It definitely sounds like granite. Thanks for your help in identifying it and introducing me to all these web resources. I'd always seen granite after it'd been cut and polished so I got a bit of an education today.
posted by jwells at 5:12 AM on January 14, 2005

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