Expert: Dr Thomas Bell - 6/14/2009

Question
QUESTION: I have a rock that my grandparents (they have both passed away) picked up while hiking many years ago (they did a lot of hiking here in New England & throughout the country and frequently picked up rocks, minerals, stones, etc. of interest, so not sure of its origin).  They gave it to my mother and I found the stone while doing some yard work for her.  It is a dense, medium grey stone with chunks of perfectly squared/geometric, copper/metal looking pieces throughout it. At first I thought it might be cement and someone was being crafty, but I work with stones (my sister is a landscaper &  I help her with stone work) and upon closer inspection, it does not appear to be man-made.  Thanks a lot, Sally.

ANSWER: Sally,

I'd be happy to help you but I need some picture.  You should take two sets, one of the dry rock and the other of the rock after you have wetted it.  At least one photo of each set should be a closeup (2-5") and another from about a foot away.  Natural light is always best.  Repost your question and upload the photos using the grey button.

---------- FOLLOW-UP ----------

QUESTION: Thanks a lot.  If it isn't too much bother, I am sending along pictures of a second rock for identification.  I don't know if it is some kind of lava or pumice stone.  It seems to be made of layers and chucks.  The black streak in the photo of this rock (I labeled the photo as 'rock B') is not a smudge or tar, but present in several areas of the rock.  Thanks again, Sally.

Answer
Sally,

Well, that helps a little bit.  I only see one picture but it is revealing.  This is not man made at all.  It is a natural rock texture.  Those hollows, properly called molds, are most likely voids that were once filled with an iron mineral.  

The molds look cubic, thus the minerals that filled them were almost certainly a pyrite crystal.  Pyrite is composed of sulfur and iron and most non-geologists would know it as "Fools Gold".  Pyrite is a very common mineral that is not too happy when exposed to oxygen in the air or dissolved oxygen in ground water.  This causes it to oxidize (rust) which forms iron oxides.  These iron oxides are rust colored which is indeed the dominant color in the photo you have posted.  It is likely that this pyrite rich rock was exposed by erosion and the pyrite oxidized leaving a void in its place and the iron oxides stained the surrounding rock mass.