Licensed Professional Geologist here, and licking is a totally valid method for distinguishing between mineral hand samples that have very similar properties otherwise. For instance, halite (NaCl, aka table salt) and sylvite (KCl) are isomorphous, which means they have the exact same crystal structure and are very difficult to distinguish using traditional methods. Halite tastes like salt, but sylvite makes your tongue tingle when licked. Our mineralogy professor told is it's the only reliable way to differentiate the two without actual chemistry analysis.