So you’ve got a rock, and you want to know what it is. There’s a lot more to identifying rocks than meets the eye, and even those of us with a long time in the field still need to be careful at times. There is no one-size meets all solution to this sort of thing, however, instead we’re left with a battery of tests and a few bewildered new rockhounds trying to sort things out on their own.
If that sounds familiar then you’re in the right place, let’s take a look at some of the ways you can learn how to identify your rock!
Let’s Talk Rocks
Rocks come in many forms, but before we even begin we’ll need to have a basic idea of what’s going on from a mineral standpoint. Many rocks are comprised of multiple different types of minerals, and some are even defined by the mix of minerals that are contained within.
So we need the basic terminology to describe what we’re seeing:
- Mineral- A single molecular compound. Calcium carbonate is a mineral, for instance, but it can take on different forms.
- Rock- A conglomerate of different minerals. Rock is used interchangeably with stone but technically refers to the formation itself.
- Stone- A piece of a rock formation.
- Crystal- An ordered arrangement of a single mineral with defined properties.
Using these terms precisely is a good idea since it helps to determine what you have in hand. A single mineral can form into many different forms, and can actually fill all of the definitions above in some cases.
For instance, one of the most common minerals in the earth’s crust is silica, SiO₂. It makes up many of the most common stones that we find and collect, but you wouldn’t be able to determine that they’re all the same type.
Silica arranges itself into the following common forms:
- Quartz- The crystallized form of silica is the humble quartz crystal, known for it’s six-sided points with pyramidal terminations.
- Chalcedony- A cryptocrystalline variety of silica, which is comprised of an intergrowth of two forms of silica (moganite and quartz). The crystals are so small they require special microscopes to observe, otherwise the material appears in a glass-like form.
- Opal- A complex mass of stacked microscopic silica spheres, lacking a crystal structure entirely. Opal is often hydrated, with large amounts (relatively) of water contained in its structure.
- Obsidian- A volcanic glass rich in iron (giving it a black color) but comprised mainly of silica. Obsidian lacks any ordered structure at all. It’s considered a glass due to the lack of structure.
It is, but it’s something that anyone can learn at a basic level. We’re not talking about comparing the refractive index of emerald simulants or sussing out the origin of a stone based on microscopic inclusions in this case.
We just want to know, roughly, what our rock is.
Once you understand basic testing you’ll find that the majority of stones are easy to identify, especially in their raw form.
Doing More Than Just Taking a Look
When you take a look at the surface of a stone, it’s giving you more information than you know.
The visual inspection of a rock includes the following properties:
- Coloration- Colors are important for identifiers, but they’re essentially meaningless without any other data on the stone.
- Patterns- The pattern of the colors can also help indicate what the mineral sample you’re looking at is.
- Luster- The exterior appearance of the stone. Becoming familiar with the basic terms can help a lot but be aware that luster is a bit subjective and different sources can list it differently.
- Grain Size- In coarse rocks, the size of the grains is a great indicator of what you’re looking at.
- Overall Form (Raw Only)- The shape of the exterior of the stone can also give hints as to what it is.
All of this adds up to give us a good impression of the stone but nothing can be confirmed without further testing. Visual identification is often enough when you’re familiar with an area but I recommend going further in most cases.
Hardness testing is one of the easiest tests to perform, and you can do it with or without specialized tools. If you read over my notes in things like our agate guide I always mention using a knife for hardness testing. It makes it easy to distinguish between similar stones in the field, common opal and massive (ie: milky) quartz for instance.
Hardness testing can be done with specialty sets that will let you precisely determine the hardness of the stone. Pick sets are great and let you quickly get results. They’re also a bit expensive, especially if you’re new to the hobby.
I’ll admit one thing: I’ve never owned a pick set.
Instead, I use the following to get a rough idea:
- Fingernail- ~2.5
- Copper Rod- ~3.0
- Nail- ~5.5
- Knife- ~6.0
- Common Sandpaper (Corundum)- 9.0
From there it’s a game of figuring out where the hardness fits. I give a more complete description of the process in our guide to Moh’s scale.
Streak testing is super simple and can really help with identifying stones.
You can use a streak plate or any piece of unglazed ceramic. You just need something hard and kind of porous. I use the underside of my coffee cup, especially in the field as I usually finish my cup in the car on my way to outings.
You can also purchase a purpose-made streak plate.
Streak testing is simple to conduct:
- Grab your rock
- Scrape it on the ceramic
- Look at the color
Some minerals can be counterintuitive in this regard. For instance, hematite leaves a red streak despite possessing a deep, shiny black as its color. Most minerals will leave a white streak, but others can give you a tell-tale sign.
It’s a quick and easy test, which makes it a favorite among amateur rockhounds.
Testing Specific Gravity
Specific gravity is a relative density measurement, which is measured in comparison to water. Rather than the usual g/cm³ formula. It’s a frequent measurement you’ll see listed with stones and it’s key to getting it right when you have lookalike stones.
You’re going to need a few things to do this:
- A Scale Accurate to .01g
- A Cup of Water
- A String, Paperclip, or Wire
- Something Sturdy to Hold the String
There are actual kits that make this whole process a lot easier. Otherwise, you’ll need to rig something to hold the stone steadily on the end of a string to get the reading.
Do the following:
- Weigh your specimen dry and write down the weight. All measurements should be in grams. This is the dry weight(DW).
- Add water to the cup and set it on the scale. Tare the scale so the reading is no 0.0g
- Either tie your string/wire or bend your paperclip around the sample to hold it firmly.
- Hold the sample in the water with your holding device and then write down the second measurement. This is your wet weight(WW).
- Divide your dry weight by your wet weight, this is the specific gravity reading for your stone.
You can now compare it to the known ranges for stones you’re looking for.
A common example in my area would be comparing nephrite jade with the more common green chert/jasper found in the same areas. With similar coloration and hardness, it can be hard to tell which is which when I find smaller pieces broken off from the original nodule.
Just do a specific gravity test.
Jasper has a specific gravity of 2.5-2.9, while nephrite jade runs from 2.9-3.02. Most of the jasper in this area is on the lower end of the scale, but even if it wasn’t you’d be able to tell the difference clearly after this test.
Specific gravity is one of the big, definitive tests that you can do to prove when you have a stone with conflicting identities.
What About Rock ID Apps?
I’m not a fan of the apps used to identify rocks for the most part. While some of them can be useful, there’s a lot that simply can’t be picked up by a camera when you’re dealing with stone identification.
They can be helpful for looking in the right direction when you have no idea what the stone is, but performing the above tests is required to actually give you a definitive answer.
I’ve tried using them on occasion, but I’ve found them to be dead wrong as often as right when performing in the field. Your results may vary, but the bottom line is that rock identification apps are often inaccurate as they rely only on the visual identification of the stone.
Where Do I Check My Data?
When I was in my early teens and without internet access I used books to help me identify stones. I still do in unfamiliar areas, usually purchasing an individual guide for the region. Just because something looks familiar doesn’t mean I’m right, after all.
These days, you can use a variety of online resources to help you figure it out.
My personal preference is Mindat’s Advanced Mineral Search feature.
I prefer it because it quickly brings up possibilities as you put in information. Mindat is an amazing website for rockhounds, and you should make yourself familiar with it.
Just enter all of the information you’ve managed to gather thus far and it will help guide you to an answer. If you’re unsure if it’s correct, then you’ll want to do some more testing to see if it falls within the outlined parameters.
Guidelines for Asking for Help Online
If you’re having trouble or aren’t sure what/how to test, then it’s time to reach out to the globally connected network of rockhounds that inhabit forums and social media. This can bring results quickly, but how accurate they depend a lot on the information that you give in the first place.
One of the first places many people go when they want to learn about a stone is social media and forums. I frequent a half dozen or so Facebook groups related to amateur rockhounding and am usually available to help. There are plenty of rockhounds with more experience than me who do the same.
However, you have to have enough information for people to work with when you’re trying to get an ID and that does mean some work on your part:
Photos come first.
You need natural lighting and try to avoid reflections. You can take shots in the shade during bright days if you have trouble keeping reflections out of the picture.
Clean your lens if the photo is blurry, or use something solid to hold your hand if you have trouble keeping your hands still. I have an essential tremor that makes taking free photos difficult, but bracing my wrist on my knee can produce a world of difference in clarity.
If you have time, take both wet and dry photographs of the stone. Wetting a rock will reveal colors and luster better, but a dry photo should be the priority.
When asking please include the following information to help identify the stone:
- Location- This is one of the most important pieces of information you can provide. Note that the location is unknown if you don’t know because the first thing people will ask is where you found it.
- Basic Hardness Test- Try scratching it with a knife before posting about it. A knife is a great dividing point to start with since it will readily scratch many minerals but won’t scratch many of the precious and semi-precious stones found.
- Streak Test Results- Include any information from a basic streak test, especially if the color wasn’t white.
If you’ve also measured the specific gravity already, include that as well. Otherwise, just note if the stone feels exceptionally light or heavy for its size.
If you take a clear photograph and include the above information (which you can record in seconds) then your chances of getting a helpful answer are enormous.
Be careful who you trust. I’ve noticed a tendency for amateurs to throw out answers seemingly at random, or insist that this piece or that piece is a precious stone or another rare object. Meteorites and tektites are two of the things I’ve seen people claim from such innocuous objects as melted beer bottles and random bits of iron ore.
Likewise in the fossil world. Fossils are awesome, but not every piece of stone that looks a bit like some part of a creature is a fossil.
The important thing is to take the information you’re given and use that to confirm the ID. This can mean specific gravity testing, inspection with a microscope, or a variety of other methods used to ID stones.
IDing your finds will take some work, but the payoff is always worth it. Whether you reach out for help or sit down and follow the knowledge on your own, it’s still important to start with good information to get accurate identifications.
Of course, don’t be afraid to reach out to these groups. People often receive good help with IDing stones with less information than I mentioned above.