Determining hardness using teeth So I know scratch tests are pretty common to quickly get an estimate of a rocks hardness. Also tooth enamel is a common material with a 5 on mohs scale. So far so good, but what shocked me is that I heard students on field trips actually use their own teeth sometimes to determine hardness. 

The obvious explanation is that I've been fooled, which I expect will be the answer. 

But if this is really true, how do your teeth survive? Or is it just an occupational hazard that you're going to need dentures before your thirties as a geologist?

Disclaimer: this question comes from a story someone told me some time ago, whose girlfriend studied earth sciences. I found it implausible then, but it keeps bugging me and I can't find information on this except that gold miners probably routinely lost their teeth during the 19th century.
No, teeth are not used to determine hardness. That would be incredibly stupid. What teeth are used for is distinguishing silt from clay.

When dealing with very fine grained rocks you might be interested in the relative proportions of clay-sized particles and silt-sized particles in the rock. This is very hard to do, but surprisingly quite easy when you try to put some between your teeth. Clays will have a smooth, paste-like, feeling, whereas silt will feel like fine grained sand. With enough experience you will be able to give a rough estimate of the proportions of the minerals.

Also notice this related question:

When can taste be used to help identify rocks?