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Well, I'm getting ready to go out hunting again and thought I would use this opportunity to learn a little more about the rocks in my area. I find tons of this stuff, I know Native Americans used these to make arrow heads. What I really want to know is why they are so porous on the outside and so hard and blue on the inside? I told you I'm a beginner :blush: any info will be great, have a nice day!post-15631-0-37348200-1410879484_thumb.jpgpost-15631-0-35466400-1410879518_thumb.jpgpost-15631-0-54595800-1410879543_thumb.jpg

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I see that there is another place listed "rock and minerals" that I should have listed this in, sorry, still learning. :blush:

EDIT: Easily moved :)

Edited by Auspex
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Rocks suitable for knapping into artefacts (those with at least some glassy texture like flints, cherts, agates, jaspers, rhyolites, obsidians etc) in their virgin form are usually only porous at a micro-level. A flint nodule given a long soak in water would only absorb a few percent of water and an obsidian nodule probably none at all. The porosity at the surface generally increases as a result of chemical attack by ground-water (usually containing acids, but occasionally alkalis). That’s when water really begins to ingress and it’s a vicious circle. More porosity allows more water ingress, which creates more weathering which increases the porosity and allows further water ingress.

The process is known as “weathering” and the change in exterior appearance (and composition) is known as a “weathering rind”. It usually takes many hundreds or several thousands of years at minimum for a thick rind to develop unless there are unusual circumstances. Ultimately, the rind may extend far enough from the opposing surfaces that it meets in the middle and the rock is then no longer what it was. Hard and shiny black obsidian ultimately transforms to soft and dull pale grey perlite for example.

Here’s a block of rhyolite (I suspect that may be what you have) that’s been split open to show the progressive weathering:


[Rhyolite (Morrow Mountain) weathering – picture from Phil Bradley's ncgeology website]

When artefacts are found which are made from weathered materials, the weathering has usually been produced during burial conditions after the artefact was made. Weathered rhyolite artefacts are common finds but they looked very different when they were first knapped. Often you can see from chipped or damaged areas that they still have virgin material below the rind.

Edited by painshill
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I keep six honest serving-men (they taught me all I knew);Their names are What and Why and When and How and Where and Who [Rudyard Kipling]

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