Seeing all these rock identification questions, got me wondering, is there no straightforward guided rock identification website?

In Intro to Geology lab, I remember going through a bit of a process towards trying to identify different rocks. There are some pretty decent cloud sites/charts around these days, not just basic descriptions like here, here, and here, but great in-depth site like here, and and especially these step-by step guides like here and here.

I know there are a lot more types and complexity to rocks than clouds, and clouds typing tends to be a little less useful (in part because the formation mechanisms are fairly non-distinct). I've slowly started to see apps to lead you through plant/creature identification increasing, which is great, but are there any places we can point people to do the same for rocks?

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    $\begingroup$ And if not, rather than answering an increasing magnitude of one-off questions... maybe we should organize some of you great geology guys to get something put together!?! $\endgroup$ Commented Jun 24, 2017 at 15:06
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    $\begingroup$ And feel free to also edit in any better cloud identification guides, ones that give more types/details or better aid the identification process. I know we frown on resource recommendations (and that's why my question is yes/no, I honestly have no idea if there's anything whatsoever for rocks beyond basic descriptions of the 3 categories, and Google didn't turn up much help). But I certainly do hold a little hope that people might stumble upon these resources when otherwise preparing to ask about cloud or rock types sometimes, and that's why the title includes both :-) $\endgroup$ Commented Jun 24, 2017 at 15:18
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    $\begingroup$ The usual term for this kind of decision-tree identification guide is "key". If you google "rock identification key" you'll get quite a few relevant results. Nothing anywhere near as comprehensive as some keys I've seen for plants and fungi, but a lot better than nothing. But, as you mention, the diversity and complexity of rocks makes this kind of thing hard to do. Even the boundaries between the three basic rock types are a little bit fuzzy. Great question, though: just thinking about why this is difficult to do for rocks produces interesting insights. $\endgroup$
    – Pont
    Commented Jun 25, 2017 at 7:33
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    $\begingroup$ See also this meta question $\endgroup$
    – Jan Doggen
    Commented Aug 28, 2019 at 7:21

1 Answer 1


I looked through your step-by-step cloud identification guides, and they seem pretty easy. The overwhelming criterion for which cloud it is is "how does it look", with the addition of the obvious "is it raining" or "do you hear thunder". After that, you only get 10 different types.

Here's why it would not work for rocks:

  1. "How does it look" is only one criterion out of many for rocks, and it is commonly the most misleading. A granite has many different appearances. Quartzite, limestone, basalt, can all take many forms. Weathering, alteration, fresh surfaces vs broken surfaces, desert patina or stream-polished can all change a rock's appearance completely. Grain size affects colour. Lighting affects colour.
  2. There are many more than 10 types of rocks. Basalt, gabbro, granite, rhyolite, dolomite, limestone, sandstone, shale, diorite, trachyte, ijolite, kimberlite, komatiite, eclogite, blueschist, greenschist, a conglomerate containing all of the above, and the list goes on and on. Some rocks may not even be rocks. Smelting slag, coal, hardened dung, a piece of concrete or any other kind of synthetic construction stone.
  3. The many other tests used to identify rocks may not be applied properly by people with little experience. Is the mark left by your knife on the rock really a scratch on the rock, or is it the streak of the knife? Is it really hard or is the scratch you just did was actually silt sized grains of quartz dislodged? What's "heavy for its size"? Everything is relative. Where a geologist sees green, brown, yellow, black or red, a non-geologist sees brown, brown, brown, brown and brown.
  4. Even professional geologists sometime disagree on what a rock is. I've seen (and been part of) so many arguments about rocks, and I have 7 years of experience in this thing. Occasionally the verdict is given by...
  5. Methods to positively identify a rock (thin sectioning, scanning electron microscopy) that are not available to the general public and probably not even to the amateur geologist.

Therefore, if a key would exist, it would be extremely complicated and pretty much useless.


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