I've got a confession to make: I lick rocks. So, in fact, do the vast majority of geologists and geology students. It is, believe it or not, often one of the most useful diagnostic tests for mineral or rock identification, which is hardly an easy task. At a party one of my old TAs attended with some other geology grad students, a non-geologist said "Geology's easy- what's there to learn after you learn all the different rocks and minerals?" Cue all the geologists bursting into hysterical laughter- there are over 5,500 known minerals, a number that's still increasing. Rock identification can be likewise perplexing at times, albeit for very different reasons.
Halite, a mineral composed of sodium chloride crystals. Halite is the most basic mineral to identify via a taste-test, because tastes just like, and is, table salt. Calcite, which looks almost identical, lacks the salty taste. Calcite actually has an incredibly faint bitter taste, but you've got to train yourself to notice it. This can be useful when looking at crystal veins in rock- the three most common precipitate crystal veins are calcite, halite, and quartz, which has no taste. [Image source]
Because of the often absurd difficulty in rock and mineral identification, geologists will use any diagnostic tools available to them, and the mouth is an excellent one- the tongue especially is a quite sensitive organ, capable of chemical analysis (that's what taste is!) and excellent texture detection. Many geologists only develop the skill to a certain point, but there are definitely benefits to developing it farther- not least gaining a bit of notoriety and a reputation for eccentricity, valuable commodities for many scientists.
One notable historical rock-licker was Francis Buckland, a nineteenth century naturalist better known for his love of animals- learning about them, collecting them, and, most importantly, eating them. He made a goal out of trying to eat one of every animal. (Moles were the worst tasting mammal in England, but bluebottle flies were even worse yet.) Buckland is really better known as a zoöphagist than a zoologist. He had, however, a lesser-known ability to identify nearly anything by taste- including rocks and dirt. Once, when lost in the fog in a carriage with some friends, he was able to successfully identify where they were in relation to London by tasting the dirt on the side of the road! (This was especially important at the time, since highwaymen were still present in England, and they ran a real risk of being robbed and killed.)
Sylvite, a mineral closely related to halite, is composed of potassium chloride. It's got a much more pungent and bitter taste than halite, though both are quite salty. It's also probably what gave the dust its red color on Crait in the Last Jedi. While it seems like it should be easy to tell halite and sylvite apart, they both have a wide range of colors they come in. Himalayan rock salt, for example, is usually halite, but often visually resembles sylvite more closely. [Image source]
Most rocks and minerals, however, have faint or no tastes. A skilled rock licker can identify a dozen varieties of salt, among other things. Coal can be identified by its taste, but since it tastes nasty, and it's otherwise easy to identify, I can't highly recommend it. Sulfide minerals also tend to have a distinct taste. There are, however, other reasons to lick rocks. First off, grain size. It's somewhat tricky to compare grain size by the naked eye even with a grain size tool, and your skin often isn't quite sensitive enough feel the differences- your mouth is, though! You can tell the difference between siltstone and shale easily this way- siltstone will feel gritty against your teeth, while shale won't.
There's another unique trait some rocks have- they stick to your tongue! This is a property of the porosity of the rock- more porous rocks will stick to your tongue. Chrysocolla is a hydrated copper silicate mineral. It's quite beautiful, but it's also entirely possible to confuse it with azurite or turquise, other copper minerals. Chrysocolla, however, unlike the other two, sticks to your tongue. There are quite a few other minerals that can be tested this way as well, like umber or the clay mineral kaolinite. One of the most important uses of the rock licking test, however? Finding fossils. Fossilized bone tends to retain the porous structure of actual bone, and tends to stick to your tongue. Fossilized wood does not- and it can otherwise be quite tricky to tell them apart. It's also a good way to tell fossils apart from mimetoliths, rocks that just fool you into thinking that they're fossils (or meteors, or arrowheads, or...) by having a similar shape. Diatomaceous shale, a sedimentary stone composed of countless tiny diatoms, microscopic sea creatures, also sticks to your tongue.
Chrysocolla can be distinguished from other, similar looking minerals by the way it sticks to your tongue. [Image source]
Another reason to lick rocks? It makes them shinier, and can give you a good idea of how they'll look when they've been polished. This is more the territory of rockhounds and lapidarists than geologists- they go for pretty rocks, while geologists, well... Rockhounds and other rock collectors often make fun of how ugly geologist rock collections are. My dad's a lapidarist, and he's always making fun of the fact that my most prized rock is a volcanic bomb- it's just a hunk of volcanic rock that hardened from magma in midair as it was being launched out of a volcano. Really cool, but not pretty at all. All that being said, sometimes licking rocks will show colors not visible when dry, allowing you to make a tricky mineral identification.
Soil scientists also regularly taste dirt. Not just to find their way back to London, either- you can tell a lot about soil by its taste and texture in your mouth. If it's slimy, it has a high clay content. Gritty, it's got a lot of sand. You can learn to tell a lot about the organic content of the soil this way, as well. Especially skilled soil scientists can often even tell a good bit about the terroir of the soil- the chemical properties that affect the taste of wine grapes and other crops grown in it. Of course, many farmers possess these skills for soil as well. Also, some residents of the American South just eat dirt. Doesn't do you any harm, and some clay rich soils can apparently taste quite good when baked and seasoned with vinegar and salt.
Autinite, a uranium mineral. Do not lick. [Image source]
Fair warning, however- there are plenty of rocks you shouldn't lick. Galena, for instance, is a lead ore, and can contribute to lead poisoning. Autinite is part of a whole class of uranium-bearing minerals- licking radioactive elements is not advised. Cinnabar, despite sounding like cinnamon, actually contains mercury, and should not be licked. Fresh igneous rocks should not be licked, as they're quite hot and may burn your tongue. Many other minerals contain arsenic and other toxic elements and compounds. A good rule of thumb is to not lick anything bright red, orange, or yellow- some of these brightly colored, dangerous minerals, however, have strong smells. (Though sylvite is fine to lick, and it's often bright red.) Sulfur smells are common, and arsenic sulfides smell garlicky. These dangerous minerals to lick are relatively rare, but caution must still sometimes be practiced- this is why rock licking needs to be part of a full suite of mineralogical identification skills, not merely an isolated talent.
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