Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Rustycat]

I've found a few beach cobbles like the following, and I'd appreciate it if someone can tell me what they are.  Of course I have an ignorant opinion, but I'd really appreciate an informed identification.  Thanks.

Pictures of same stone, one with it being back lite by an LED light.

49931067516_5c718b64cd_c.jpg (69.65 kB . 600x800 - viewed 380 times)   


49931067516_5c718b64cd_c.jpg (69.65 kB . 600x800 - viewed 380 times)

[Rustycat]

Guess, I'll try and post the picture with the back light.

[Rustycat]

Well, guess I'll post this link: http://www.jsjgeology.net/Ophiolite-rocks.htm

I kind of figure this rock falls into this category with chromium and manganese providing the green.  But, as I said, I'm really ignorant, so, would certainly appreciate some opinions.

[ToTheSummit]

Sorry, I'm no help, but I like it.  Great color coming through with that light.

Once Frank (Lithicbeads) checks in he will likely be of some help.  He's somewhat an expert on rocks of that area.

[hummingbirdstones]

Wish I could help you out with an ID, but I've got no clue.  The color of it backlit is amazing, though!  Great find.   

[lithicbeads]

It is serpentine with a weathering rind. Fairly typical for beach serps. The color is from iron.

[Rustycat]

lithic,
Thank you for the response--I hoped you would do so!  I was leaning towards Olivine, so happy to have a definitive answer.

And doing some brief reading, it is now my understanding that Oilvine metamorphoses to Serpentine.  And, I really didn't know that Serpentine  could occur as a colored clear stone, but searching I found polished and faceted Serpentine stones with a high degree of translucency. Thanks again.

Back in January, I sent to a local talk at our Library on the Rocks of Vashon. There were 3 volunteer geologists presenting and identifying rocks people brought in to be identified.  They had a sliced specimen similar to the one I posted and were uniform in their opinion that it was "Jade".

[lithicbeads]

 Most geologists have never seen a piece of in situ jade and until a couple of decades ago did not want to recognize the existence of rodingite , the suite of rocks which our west coast nephrites are formed in. The serps in Washington are from olivine to an extent but the vast majority of the serp precursor rocks  are not olivine. A more common serp on the beach has a lizard skin like surface and is black with chatoyant flecks below the rind. This type comes from Deer creek and it's volume is huge so lots makes it to the beach.Olivine serps are quite rare simply because the serp zone around  the Twin Sisters range next to mount Baker has a very thin serp reaction zone averaging about 100 feet thick. The type of serp you have is mostly found in tectonic zones in the north Cascades ( in major fault strands) and within those tectonic zones it cause a lot of havoc as it is less dense than most of the rocks in the zone and the huge masses of serp  rise within the zone .The faults spin the rocks within the fault zone as one side goes east while the other goes west for example while the huge masses of serp mix rocks vertically as they rise.The fault zone that most of that  type serps come out of in the sound mountains is the Darrington -Devil's mountain fault zone and  along much of it's 50 mile plus length it's tectonic zone is over a mile  wide and in spots much much wider.

[Rustycat]

lithic, wow and more wow.  Your knowledge is stunning.  I read and reread on this amazingly complex environment, and am convinced I'll need to spend a lot more hours pencil and paper in hand to incorporate a small part of the understanding that you so readily display.

[lithicbeads]

 I will post pictures of olivine this evening. If you go to the Mosquito lake bridge over the middle fork of the Nooksack river you can find lots of it on the gravel bar. Lots of good parking but sketchy people tend to live there. If you cross the bridge and drive downstream a mile or so  where the river is at the level of the road you can park and walk to the gravel bars on that side. I have never had a problem on that side .

[Rustycat]

Just thought I'd post a pic. of some more Puget sound beach cobbles that I've cut.  I assume they are mostly metamorphic. [ Invalid Attachment ]