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[drnihili]

An odd rock tonight.  If I had to guess, it's quartz crystals grown in basalt.  It's softer than most of what I find, a knife will scratch the base rock, it cuts quickly compared to what I'm used to.

Anyone care to school me on what this might be?  The firs shot is an end that's about 1x2 inches.  The second is a closer shot of the same piece, hoping to get a better look at the structure. 

[Enchantra]

Looks like what is called Chinese Writing Stone.
Frank might have a better idea of what it is. 

[lithicbeads]

Those are the feldspar phenocrysts we se in Cinese writing rock. The dark matrix however is not mormal as it is normally quite hard and can vary from a diabase to a basalt. These typically form at the bottom of volcanic melts , areas that have been hot long enough for constituent migration and the formation of crystals. I suspect that yours formed from the last fractions of the melt  and that the ingredients of the matrix rock had been deprived of silica. It is possible for silica to be leached from whole rocks at once at times and this could have happened also with the feldspar being relatively impervious because the chemical operation involved did not have either the proper chemistry or perhaps enough energy to involve the feldspar. Folks good at thin sections could writes boks about this rock I am sure . Good catch.

[drnihili]

Nice!  Thanks!  I love learning what I've found and now I have leads from which to go learn more.

Frank, I'm understanding you to say that this rock might be a good one to try a thin slab on.  I took a slab from it last night, but not a thin one.  I was debating trying to get one thin enough to to have the crystals span the depth and allow some light transmission.  I'll have a go at that on that later and post back how it turns out.

[Itsandbits]

Nice!  Thanks!  I love learning what I've found and now I have leads from which to go learn more.

Frank, I'm understanding you to say that this rock might be a good one to try a thin slab on.  I took a slab from it last night, but not a thin one.  I was debating trying to get one thin enough to to have the crystals span the depth and allow some light transmission.  I'll have a go at that on that later and post back how it turns out.

I don't think you can get a thin section of this that will stand up to anything; I think Frank was saying for analysis. The grains in the weaker basalts won't hold it together very well, even thicker pieces will be brittle in even hard basalt. All the weaker basalts have been turned into silt by glaciers and other erosive conditions if they were exposed to them.

[Enchantra]

I don't think you can get a thin section of this that will stand up to anything; I think Frank was saying for analysis. The grains in the weaker basalts won't hold it together very well, even thicker pieces will be brittle in even hard basalt. All the weaker basalts have been turned into silt by glaciers and other erosive conditions if they were exposed to them.

I agree.  In order to cab this I would try something no thinner than 1/4 inch for a slab.  Even that after testing the grinding on your wheels you may find it needs stabilization.  Really thin sections are more for analysis of it's constituent parts.

[drnihili]

Fair enough.  I'll give it a try but not expect too much.  As it is, I don't think it will work for much. Unless you get a pretty high intensity raking light (which just coincidentally is what I've been photographing under) there isn't enough contrast between the crystals and the background.  The crystals are transparent with just a hint of color, so without specific lighting you mainly see the rock behind.  Sort of meh to look at.  I suspect this is a rock that is more interesting than beautiful.  That's ok with me, though.  My wife is the artist, I'm just the enabler and gopher.  To me, the beauty is that I now know more than I used to.

[Minkos61]

Google Chinese Writing Stone some very nice images can be seen even some nice cabs.

[drnihili]

I've got to say, it's a royal bugger to put a polish on.

[Itsandbits]

I've got to say, it's a royal bugger to put a polish on.

yeah, it' looks like pretty soft open basalt. You need a real fine grained material to polish. Nothing wrong with a matte finish though :)

[lithicbeads]

I think you have had great advice. Are you starting to see process when you look at certain rocks?

[drnihili]

I think so, Frank.  What threw me on this piece what the large crystals against the finder grained background.  Almost all of the rocks I've been looking at least somewhat homogeneous with respect to grain size.I could see an odd distribution of crystals on the outside of the rock, but didn't expect them to continue throughout.

I've cut a section about 1/16" thick.  It seems pretty sturdy in the hand, that's no guarantee it would survive buch grinding though.  It is a bit more interesting as some of crystals now go all the way through and so transmit any backlighting.  I may work with it a bit more just for fun, or I may get distracted to other things.