I've found a strange black / green / shiny rock washed up on Sheerness beach. It's attracted to a magnet, albeit relatively weakly and in differing amounts across it. It has thin seams of what looks like pyrite and it has lots of varying sparkling dots of what looks like metal all of over it apart from the blacker areas. It also shows some iridescence in small areas when a light is shined on it.
Any ideas on what it may be? Man made maybe?
Looks like the labradorite I found recently on the beach in Dorset. Does it show more colour when you wet it?
Thanks for the reply!
It doesn't show much more colour went wet, it contains a lot of metal and a magnet responds to it. There are what look like seams of gold coloured metal and what appear to be metal flakes. I think the greenier bits are maybe olivine.
My iphone photo's don't show the metallic sparkle very well.
It looks basic or ultramafic.
The greenish colouration might be due to serpentinization (but olivine is also a possibility).
Magnetite has been described in serpentinized ultramafic rocks...
It would be useful to know its density...
At Sheerness, this will be exotic; could have been transported by man.
Thanks for your reply Mike,
It's density is approx (my measuring jug is not the best) is 2.9 / 3
Thanks again for your help in trying to ID this strange but beautiful lump of rock.
2.9/3 is about right for mafic rocks (ultramafic would be a bit more).
So it will be something like a gabbro (going on its coarse grain size).
The iridescent patches probably are due to labradorite (note that that is a mineral not a rock, and it is a bit less dense, at 2.7). The presence of labradorite supports the gabbro idea. ...In which case, the greenish parts may be olivine, but could still be due to sepentinization. Disseminated magnetite has been recorded from gabbro. In fact, the St Peter Port gabbro (Channel Is.) includes magnetite, pyrite, haematite, pyrrhotite, as well as olivine (see also https://www.smenet.org/opaque-ore/plate05f.htm).
So I don't think I'm going out on a limb in IDing your specimen as gabbro.
Thanks for taking the time to ID it for me. I'm no geologist and just looking into what this simple find might be has been really interesting. This forum is a fantastic resource!
It does look very similar to the some of the photo's you've attached. Although mine does appear to have more of a metallic shine, does that mean it's just more got Labradorite?
Yes - looks like high % labradorite.
I'm sure it would show its magic nicely if cut and polished, like this
I have collected quite a few, I believe, of these in one place in Essex. I also believe these to have been made as a rather stunning looking tool.
Most of my pieces I would describe as you have above verbatim, it was uncanny to read.
I am getting some of mine looked at by a local museum - I will let you know.
Possibly eroded from some "Rock Armour" - any nearby? Larvikite from Norway is very popular...