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[drnihili]

So this rock came from our yard (we have an interesting yard) and I was quite surprised to find that it was colonial blue inside.  One of my main interests in rockhounding is learning about the rocks around me.  In that vein I have two questions:

1) What is it?
2) Aside from asking on a forum, how would I go about figuring this out for myself?

I know to check for hardness (a good steel knife won't scratch) and I know that location is important.  Of course, where I'm located the rocks can be drug in by glaciers, and a lot of the rocks we find were collected by the river that ran through our yard about 100,000 years ago, so we get quite a variety.

Clues and tips greatly appreciate.

-Jim

[lithicbeads]

You are unlikely to ever find a similar rock in a guide book because guides have to show the norm of a rock type not a variety of the type. Geologists even state in their dissertations that certain rocks in their area are so metamorphosed that they are not definable only describable. Testing can only tell you the constituents not the rock type that the constituents as a whole represent. Having had access to lots of analytical testing of Washington rocks recently I can assure you that experience is still important.Blue is very rare so this rock is unusual. I will bring a hardness test kit and a loupe and we should be able to determine which type of rock was the primary type and which was metamorphosed due to intrusion. Normally rocks that are  differentially intruded has a very high quartz content . Brittle fracture is the key to making pathways for fluids to change the rock and  quartz is least ductile rock around. Again this probably came from a contact of the Mt. Persis volcanics and the adjacent granites.Field geology is still an art to an extent. Most geo departments are no longer interested in thin sections and field analysis, rather analytical techniques that fit the needs of the oil world are now paramount. That world was interested in trace fossils in limestone for the best part of 75 years as an oil indicator but today brutal methods are the norm and we get horrors such as explosive testing in the ocean that kills marine mammals at distance. Just another aspect of life where humans are coincidental to the process. Progress.

[lithicbeads]

The relative proportions of the colors in the rock are important. From the picture it seems that the blue color is the dominant interior color while the opposite is true for the weathered exterior. This would indicate the blue is softer  than quartz.

[iceopals]

I think you should name it "Killer!".  That is an awesome piece, and I hope your yard is just chock full of it!   Cut some!!!   

[drnihili]

Frank, thanks for those insights.  That's the sort of knowledge I don't have and would love to have.  The sad thing is that I come from a long line of geologists on my mother's side, but I never really appreciated the science and so didn't pick up the background knowledge I easily could have.

I've got a loupe, 60x maginfier, and my grand-dad's old binocular field microscope, so we should be good on that front.  Teresa thinks she's got a hardness kit still in a box somewhere, but if it's not found soon, I'll probably just spring for another.

[Minkos61]

Very nice surprise for you when you cut that open I bet !!

[Enchantra]

I say that you have something special there no matter what it is. 
Go ahead, perhaps attempt making some cabs from some of it, keep a chunk for display if you can. 

[drnihili]

Ernie, Yes, I've been picking up rocks that look interesting or just different from the norm.  This was one of the latter.  I had pretty much figured it wasn't worth cutting until I looked at it with a loupe and noticed some spots that looked like fractures in quartz, sort of like a tiny flake that hasn't quite detached.  So, I decided, what the hey, and fired up the saw.  Unfortunately, I had a saw issue before I got all the way through, so I tapped the top with my Leatherman to break the flak off.  Big surprise!

Amanda, the rock seems to have some pretty significant fractures, so I'm not sure how well it would do for cabbing.  I'm guessing y'all have ways of dealing with that, but I don't know them yet.  Fortunately, Frank is coming over on Monday and can give me some excellent advice on what to do.  I'm definitely keeping some for display.

[Enchantra]

You might be able to stabilize whatever you decide to cut.  A lot of folks recommend Hxtal for such an application.  I would do some research on the process, but you could set up a simple vacuum system fairly cheap. 

A while ago Gina posted this on another forum.  Very helpful.
https://www.youtube.com/watch?feature=player_embedded&v=-DfZXM2B51Q

[drnihili]

Frank says this is oceanic chert.  The blue color is odd, but apparently not to uncommon to the north of us.

[Enchantra]

As long as it's not full of fractures a chert will cab up fine.   

[lithicbeads]

It is quite fractured. These blue cherts come in ton sized pieces about 75 miles north. Now to find something that color in a size somewhere in the middle.This is an oceanic chert from a well known deposit upstream. Maybe we should go visit the area next summer.

[drnihili]

Frank, anytime you want to go out for a wander, count me in.

I think my plan with this piece is to cut it as a display piece by cutting out a chunk down to about halfway below the already cut face.  The should leave lots of blue showing.  This of course assumes that the whole will stay together through the process.  If I'm very lucky, the fractures will leave a solid piece of a size to be turned into a bead or focal by Teresa.  If I'm unlucky the whole thing will disintegrate when I try to cut the chunk out and I'll have a bunch of pretty blue stones to toss in a tumbler.

[lithicbeads]

I don't think it will disintegrate but the cracks will have to be filled or they will fill with stone dust as you sand and polish the faced piece.

[drnihili]

What do I fill it with?  Or should I be looking at the stabilizing the whole piece as per Amanda's suggestion?  If I did that, would I likely get some cab material for Teresa to play with?