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Jan 10 2013

Obsidian

UPDATE:  One commenter wanted to know what I do with obsidian beads.  So I’ve added a photo of one of my beaded necklaces at the end of the post.  It’ll be up for sale at http://www.etsy.com/shop/gemmyjoy in a week or two.  /UPDATE

Rocks are my friends.  I especially like the ones I can pick up, look over in my hand ( maybe with a handlens) and say “this is cool!” If it’s something that somebody can make beads out of, so much the better — I’m a beader.  Obsidian qualifies.

Obsidian is an extremely felsic (feldspar/quartz-rich) lava with a chemical composition similar to rhyolite. Think silica-rich.  Really, really, really silica-rich.  But while rhyolite congeals into microscopic crystals, obsidian doesn’t, and has a glassy composition.  For a long time, the “accepted wisdom” was that obsidian cooled too quickly to crystallize.  Anyone who’s ever seen an obsidian dome next to a rhyolite flow (there’s one at Medicine Lake, California) can tell that explanation is a bunch of hooey.  I haven’t kept up with the literature in recent years about new theories for obsidian formation, so if anyone can point me to a paper in the comments I’d be grateful.

There are obsidian domes all over the eastern and northeastern parts of California, my home state.  Most of these are associated with what are still considered active (quiescent) volcanoes.  The biggest obsidian dome I’ve seen, though, is the Big Obsidian Dome at Newberry Caldera in Oregon.

So I’ll start off with some Newberry pics:

Big Obsidian Flow

Big Obsidian Flow

Here’s a shot of the iconic Big Obsidian Flow from Paulina Peak, with East Lake in the background.  There are better pics on the web, but I like to use my own photos; that way I don’t have to worry about infringing on anyone else’s copyright.  Big Obsidian Flow erupted about 1300 years ago.  Note that it doesn’t look black, like obsidian is “supposed” to look; nor is it covered in snow, because this pic was taken in July 2003.  Obsidian flows are pumicy.

There’s a trail leading up and into the Big Obsidian Flow, and the day we visited there was even a docent at the top, answering questions.  Very cool.  So we wandered up the trail, looking at the mix of black obsidian and gray pumice, with ever-clumsy me trying hard not to cut myself on any exposed sharp surfaces.  Here’s an example of what the rocks along the trail look like, with reluctant spouse for scale:

Mike along the trail

Mike along the trail of the Big Obsidian Flow

Lumpy stuff, not very picturesque, with sharp edges that will gladly send you rifling through your purse/backpack for Band-Aids or worse.  But then there are sights like this one:

Obsidian with tree

Obsidian with tree

Gorgeous, black obsidian with white streaks, topped by a little tree trying to grow in the flow.  I was absolutely charmed by this picture.

But so far we haven’t seen any hold-it-in-your-hand samples.  So I’ll offer a couple from our collection.  Note that these were NOT collected at Newberry; Newberry is a national monument and I believe that collecting is prohibited.  Our samples came from domes in California on National Forest Service land where collecting for personal use is permitted.

Obsidian sample

Obsidian sample

This piece is about two feet long.  The blue color is an artifact of the photographic lighting; the piece is quite uniformly black, with a few lighter streaks.  In the upper-right side of the picture you can clearly see the conchoidal (glassy) way the material breaks.

pumicy obsidian

pumicy obsidian

Here’s a more classic sample from an obsidian flow.  A bit of black shiny goodness, with a lot of inclusions and pumicy “stuff”.  This piece fits comfortably in your hand, and doesn’t even have many sharp edges, either!

I did mention beads in the beginning, didn’t I.  So I must show you some from my embarrassingly extensive collection.

Snowflake Obsidian Beads

Snowflake Obsidian Beads

The White inclusions in snowflake obsidian are cristobalite, a high-temperature version of quartz. These beads are about a centimeter and a half in the long direction.

Mahogany Obsidian Beads

Mahogany Obsidian Beads

Mahogany obsidian has red inclusions. I’m guessing iron is involved, but I don’t really know. They do make lovely beads, though, and I find myself using them often when I’m beading.  These are 10 mm across.   Both the snowflake and mahogany obsidian beads are from China.

So — more, and yet less, than you probably every wanted to know about obsidian.

Karen

UPDATE:  Since one of the commenters asked to see my beadwork, here’s a necklace, about 28 inches long, draped over that long piece of obsidian you saw a few pics above.

Mixed obsidian necklace

Mixed obsidian necklace

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The snowflake obsidian is cut into to star shapes, which actually works for such a dark rock. The beads between the snowflake and mahogany obsidian are glass. I always string on plastic-coated, 49-strand steel wire; that allows a natural, relaxed feeling to the finished piece, while still being resistant to wear from burrs that occur in the holes of rock-based beads. I always finish with gold-filled or sterling silver crimps, too, on these long pieces.

13 comments

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  1. 1
    rq

    The enthusiasm of your spouse is palpable… I mean the lack of. ;) Can you ask him to smile next time?
    I also like the super-creative, flowery-prose name for the big obsidian dome…
    Snark aside, thanks for the post! :)

  2. 2
    donnaoakes-munro

    My hubby and I honeymooned in Yellowstone, and did some back-country canoeing. Since I didn’t have river shoes, I had bought a cheap pair of canvas sneakers as stand-ins. While lining the canoe up the Lewis River to Shoshone Lake, the obsidian pebbles and shards in the river chewed them to tatters. Ah, memories!

    1. 2.1
      Karen Locke

      One of my fellow graduate students was vacationing in Oregon and fell into an obsidian flow. Never did hear the details, but I expect there was a lot of blood mopping up to do.

  3. 3
    hexidecima

    one of my favorite stones.
    from wiki: “Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color. Very few samples are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian) or an iridescent, rainbow-like sheen (rainbow obsidian).”

    Sheen obsidian is a definite lust object of mine and I have a lovely big cab that really needs set into something. Examples: http://www.mineralminers.com/html/obssphs.stm

    1. 3.1
      Karen Locke

      I read the wiki piece, and the bit about magnesium contributing to the dark color puzzles me; magnesium usually makes things green, not black. So I decided to stay away from discussing the color issue, except as it pertained to the beads.

  4. 4
    heliconia

    I like this post format – introducing one rock/mineral type at a time. Question – does the Big Obsidian Flow (haha, love the name) count as ‘a’a? I’m not sure if the classification is based on rock type, or shape, or both.

    My grandmother used to have a rock tumbler; I don’t think she ever made beads with it, just some nice smooth shiny stones or which I have some fond memories. Seems to be a bit of a lost art these days.

    1. 4.1
      Karen Locke

      I don’t know if this qualifies as aa. Technically, aa describes a blocky basalt or other mafic lava flow.

  5. 5
    mzann

    I like to bead too. I hope you have a follow up post showing us what you did with those lovely beads.

    1. 5.1
      Karen Locke

      See the update.

      1. mzann

        Ohh how pretty!!! I like what you’ve done with them. It’s interesting to know how you do your beads. I was taught to use string, preferably silk for that relax fit. Now I need to try that plastic coated 49-strand steel wire.
        I’ve only recently started to use the gold and silver crimps to cut down on the knotting and really enjoy using them. The things they come up with always amazes me.

  6. 6
    lockwooddewitt

    That flow in Newberry truly is a sight to behold, both from a distance and from the trail- such different experiences! A place I recommend for collecting is Glass Butte, between Bend and Burns on Highway 20 in Oregon. Huge variety of types, from standard black to some really nice black and clear stripey, to mahogany and a variety of sheen types. It’s BLM land, so collecting is allowed for non-commercial activities without a permit. You’ll want to check the regs, but I think the daily limit is 20 pounds- which is way more than most people are going to want. This spot in particular is a good one: http://www.flashearth.com/?lat=43.553537&lon=-120.006788&z=16.3&r=0&src=msl

    As far as its source/origin, the last I read is that it’s due to a rhyolite melt with unusually low water content. Water allows mobility of ions in a melt, and a lack of it prevents the ions from reorganizing into crystals. I think another thing I’ve seen discussed is that pre-eruption degassing likely plays a role. I’ll see if I can find a good reference.

  7. 7
    Tsu Dho Nimh

    And the “Apache Tears” … obsi8dian droplets you can see through, almost, kinda sorta.

  8. 8
    rq

    Lovely work!

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