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!

• I’m a structural engineer, but I work closely with geotechnical engineers and do my best to understand it as well