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Rock Identification
(imgs.xkcd.com)
'Is it worth anything?' 'I dunno, is the answer to that question worth another $5?'
Believe it or not, this is a legit thing. I haven't done anything much with rocks*, but here's a good example for soil. If it's fine grained (smaller than sand) and you can't see the particles by eye, you can rub some on your teeth:
Buttery texture --> clay Gritty on the teeth --> silt
Not approved for use on contaminated ground!