Q: I live in Toronto, Canada and am looking to gather 60-80 basalt obsidian rocks about the size of 2 fists together. Someone told me I should drive to Idaho and that I would be able to gather them from the farmers' fields. That's a very long trip for me... wondering if you can suggest a closer place or another kind of rock that can withstand high heat and not crumble too quickly, Someone brought us volcanic rock from Iceland and it's working beautifully!!
Thank you so much!
- Michaela O
A: Obsidian is a glassy, naturally-occurring volcanic rock found in places like Medicine Lake volcano in northern California, and elsewhere in the western United States where volcanic activity is ubiquitous.
Obsidian is formed when a high-silica volcanic melt (such as a rhyolite) is quenched rapidly by extrusion into water. There is not enough time for quartz crystals to form, and the result is typically a black to dark green, conchoidal, glassy rock. If you are not careful handling it, it can easily cut your fingers.
Obsidian weathers like any other rock - slowly, but it still weathers. This process is called devitrification, and you can see it well developed already in Roman-era glass. You live in Canada, in the the middle of the Grenville craton, the oldest rocks in North America, so the chance of finding un-weathered obsidian there is very small. Anything on the east side of Canada or the US will be equally unlikely to host obsidian. Iceland is nearly 100% volcanic in origin, and there is a lot of water present in lakes and the surrounding ocean, so you will find it there. I've been to Craters of the Moon National Park in Idaho, and have never seen obsidian there. This might be because the volcanism in the Snake River Plain is in the "millions of years young" range, however it could also be that I was just never in a location where volcanic extrusions encountered ancient lakes. There are places along the Cascades volcano chain from Medicine Lake, California, to Mount Baker, Washington, where you can find occasional flows that poured out under lake water - especially at Mt. Shasta and Medicine Lake volcanoes. Many of these places are national parks, however, and it is illegal to take rocks from them.
Obsidian has been used experimentally by some surgeons to make incisions that are cleaner and less ragged (microscopically) than can be made using steel scalpels. I'm wondering if that is why you want so many samples of such a specific size?
Hope this is at least a little helpful.