Help me identify this rock.

December 14, 2012 11:03 AM

Help me identify this rock!

I figured this is one of those ask-metafilter-and-have-an-answer-in-ten-minutes situations.

http://imgur.com/a/h89s6#0

I had a student show up with this rock and ask me if I could tell them what it is. I have a passing interest in geology but this isn't something I've seen before. My first guess was some sort of halite but it seems too soft (the thin crystal pieces break easily and are scratched with a fingernail). It has a satiny feel to it when you rub along the grain.

Thanks in advance, you guys are awesome.

posted by _DB_ to science & nature (9 answers total)

Where was the rock found?

Is it salty?
posted by erst at 11:08 AM on December 14, 2012

Looks like Selenite.
posted by humboldt32 at 11:09 AM on December 14, 2012

Student couldn't tell me where it was found.

Not salty - that was the first thing I tested when I thought it was halite.
posted by _DB_ at 11:10 AM on December 14, 2012

gypsum?
posted by Calicatt at 11:11 AM on December 14, 2012 [1 favorite]

... a form of gypsum.
posted by humboldt32 at 11:12 AM on December 14, 2012

my money's on gypsum too.
posted by mukade at 11:17 AM on December 14, 2012

Might be ulexite, aka "TV Rock". It's very easy to tell because of its natural fiber-optic properties -- if you place it over, say, a piece of newsprint the image will be projected to the surface of the mineral well enough that you can often make out the words.
posted by Scientist at 11:20 AM on December 14, 2012 [1 favorite]

Gypsum (satin spar) seems to be correct. Thanks mefi!
posted by _DB_ at 11:39 AM on December 14, 2012

selenite
posted by FlyByDay at 7:15 PM on December 14, 2012