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Spurred on by the discussion on my earlier question about whether or not 'identify this rock' questions should be on topic and the appearance of our first such question; I propose that we make a guide so that we can refer people to it when appropriate.

I'll start a community wiki answer to this question where I'll put in a few of the basic things that I know about. I'm not a geologist though, so I'll need you guys to fill in most of the details. We should try to rely on tests that the average person can do reasonably quickly in their own home.

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How to ask an "Identify this rock" question

1) Post a well-lit, sharp photo with a scale

Take a sharp photo in bright white lighting next to a scale or ruler of some sort. Try to use daylight (but not direct sunlight) or bright white fluorescent lights. No flash. Make sure that the rock is well lit but don't saturate the image. If possible, use a plain background, such as a sheet of white paper. Also remember to either get the units of your scale in the picture or post it in the question. If the rock has a visible crystal structure, make sure that it is clearly visible in the photo.

Note: while a good photograph is important, it is not a substitute for a written description. Images of unknown things cannot be searched. For search purposes, questions should provide an image, and also describe the image in as much relevant detail as possible.

2) Describe its properties

What color is it? What kind of lustre does it have? Is it made up of layers? Can you see grains? How easy is it to break pieces off? How homogeneous is it? Does it seem unusually light or heavy for its size?
Name its main property (color, structure) in the title (as suggested here).

3) Describe where you found it

Be as specific as you can. What part of the world? Was it on a beach? Did you find it lying on top of the ground or did you have to dig for it? Were there lots of them around or was this the only one? Note that if you got the rock as a gift or you bought it, you won't be able to provide a location, so your question will most likely be closed as "unclear".

4) Test its hardness

Test the mineral’s hardness on the Mohs hardness scale. This is pretty easy to do by comparing it to some common household items. The list below gives the hardness of some common objects; if these objects can scratch your rock, then the rock is softer, otherwise it is harder.

  • Fingernail: 2.5
  • Penny or other US coin: 3
  • Knife blade: 5.5
  • Glass: 5.5
  • Steel file: 6.5
  • Quartz: 7
  • Diamond: 10

5) Be prepared to answer follow up questions

More than likely some more information will be needed to identify your rock. Users will post clarification question in the comment section of your answer. If the questions only need a one line answer, then leave it as a comment. If the question requires a longer answer, then edit your post to include the additional information.

6) Tag your question with

This tag will help identify your question as a identification question and make it easier to find for people that can answer your question.

7) Name your question in a relevant way.

"Help me identify this rock" or "Rock identification needed" are very unspecific, and won't help you get good answers. A title like "Rock ID: soft, white, from Dover, UK" will help your question to stand out, and will also make it more interesting to experts in the region, who might have a better idea about the geology of their area.


Why was my question closed as "unclear"?

If your question doesn't address the points above, it makes the question vague with many possible answers and low confidence. If you haven't responded to comments and have not provided enough information, your question will likely be closed as unclear. If this happens, please edit your question to include the things above, which will automatically nominate it be reopened.

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    $\begingroup$ Diamond is a common household item? :-P $\endgroup$ – Semidiurnal Simon Apr 21 '14 at 21:26
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    $\begingroup$ @SimonW Well, many women have one on their fingers :). It still isn't that useful though since it will just scratch everything except another diamond, lol. $\endgroup$ – Chris Mueller Apr 21 '14 at 21:28
  • $\begingroup$ Great guide. Could maybe add to the first point that the photo should be framed such that the crystal structure is visible, if possible. $\endgroup$ – naught101 Apr 23 '14 at 0:06
  • $\begingroup$ @naught101 I made it a community wiki so that anyone could contribute to it. Feel free to improve it as you see fit. $\endgroup$ – Chris Mueller May 1 '14 at 15:21
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    $\begingroup$ Would be cool to have "do"s and "don't"s example pictures if anyone has them. $\endgroup$ – Leo Uieda May 14 '14 at 1:45
  • $\begingroup$ @LeoUieda: I think that is a good idea, but it should be in a separate answer. This is nice and concise as-is. $\endgroup$ – naught101 Jul 9 '14 at 13:50
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    $\begingroup$ Would "describe its properties" include actually measuring weight and volume? Would it be wise (given that we don't know yet what it is) for the OP to measure its actual volume by submerging it in water? We could then calculate the actual density. $\endgroup$ – Jan Doggen Jan 26 '16 at 16:12
  • $\begingroup$ And another addition: would taking a photo in UV light help? I don't know anything about rock identification, but if you happen to have a source of UV light around (and I don't know which ones, frankly - I have two different wavelengths UV lights because I happen to collect stamps), would it help? Or other kinds of lighting? $\endgroup$ – Jan Doggen Oct 15 '16 at 11:54
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A streak test is another test that can be useful in mineral identification. If you are able to perform this test, include the results in the question.

Streak is the colour of the powdered mineral. In many cases it is the same colour as the rock but in some it is very different from the normal surface.

To perform a streak test, you need a piece of hard, white, unglazed porcelain, ideally with a slightly rough surface. Geologists use specially made pieces called streak plates but you can often use the bottom of a cup or plate. If the bottom is glazed, use the unglazed foot-ring if it has one. Be careful that the pot is harder than your mineral or rock, otherwise you will just see the powdered porcelain (look to see if you have scratched the pot).

Rub your mineral on the streak plate to produce a powder and describe the colour. Some minerals with diagnostic streak are:

pyrite: Brassy-coloured surface but produces a grey streak.

hematitie: Often grey colour but produces a red or reddish-brown streak.

goethite: Often grey colour but produces a pale yellow streak

ilmenite: grey to black producing a black streak

flourite: May be white, green, purple, or clear but produces a white streak

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Describe and photograph a fresh surface.

Breaking open a rock to see an unweathered surface is very important for identification. In some cases the minerals show up better on a weathered surface so include both pictures if possible.

This isn't as important for mineral crystal specimens and we understand if you are reluctant to destroy your sample but not seeing a fresh rock surface will make it hard to answer your question.

Look at and photograph the surface when wet and dry.

Geologists often lick or spit on rocks to bring out features that are hard to see on dry surfaces (you could also use plain water). Do not lick rocks that might be toxic like some ore samples.

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They are little differences for the four posible cases: mineral, rock, fossil, antropogenic.

In the geological point of view, what a geologist needs is a good picture and a precise location. If he can make a picture of the outcrop better. If the poster tells me his location, some countries has not only an application to know what geological unity is, but sometimes a cartographic map and a pdf where the geologists explain the local geology. This can lead in some cases to a inmediate identification, whereas discussion uses to be between two or three posible minerals/rocks.

eg someone posted a picture from north of Ireland. I can take a look at Ireland Geological Survey Then collectionists may not be very interested, but it can not only help to determine the piece, but to give an age (specially important in paleontology), and write a little geological history for the students.

It migth be interesting to add things at point 3. url on maps. or geological unit if known. Picture of the outcrop if possible. And then, for minerals and fossils it is not a good idea, but for rocks, fresh surface picture matters as said.

For a rock it migth be fine, it they have an augmentation glass, to tell them to describe what minerals they see, but I think this is too much to ask them to do if they are not geology related. I don't know how much far can we see texture if the picture is high quality and allows zoom; for a rock this is esential. If they put a rock and a bad picture and say "Hey, what's this rock?". "Yeah that looks a sandstone" we can say them, but to classify it you need to see the quartz grains. Size, rounded, translucents, matrix?. With the pictures they use to provide it is completely impossible to classify correctly a rock.

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