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:
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:
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:
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.
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.
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.
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 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.
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.
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.