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A scratch test is an essential method of mineral identification. It is widely used because of its simplicity. A scratch test is based on the Mohs scale – a relative hardness of minerals scale consisting of 10 index minerals.
A scratch test is a determination of a mineral’s hardness by scratching it against a tool or other mineral with known hardness. A scratch test is based on the Mohs scale of minerals’ relative hardness. Scratch test can be easily done at home just following 8 simple steps.
Geologists commonly use a scratch test in the field. This method is rapid. It gives an understanding of what group of minerals do you work with and what are further tests should be performed for 100% mineral identification. You will learn how to conduct a scratch test easily due to our 8 steps guide, with insights from professional geologists.
If you are interested in checking out the best Mohs hardness test kits only you can find them by clicking here (Amazon link).
What Particular Property is Tested Using Scratching Method?
The relative hardness of minerals is a property that tested using a scratch method. The harder mineral will scratch the softer one. The Mohs scale of mineral hardness consists of 10 minerals used for the scratch test. Also, some tools like glass or a knife with known hardness can be used for this test.
What is Determined by a Scratch Test
The hardness of minerals is determined by a scratch test according to the Mohs relative hardness of minerals scale. You can determine how hard the mineral is by a scratch test. A scratch test determines whether your mineral is softer or harder than the tool or index mineral you use for the test.
When Do You Perform a Scratch Test on a Mineral
A scratch test should be performed during every mineral identification. A scratch test is essential as it can easily differentiate between two similar-looking minerals.
Also, it is a very easy and expressive method. Scratch test performance can greatly speed up the mineral identification process even at home.
How to Do a Scratch Test on Rocks
A scratch test can be easily done at home with a list of simple tools or with several known minerals. It is a quick method for the determination of mineral hardness and further mineral species identification.
To do a scratch test on rocks, you will need a list of simple tools with known hardness (fingernail, a piece of glass, a knife). Scratch your mineral with a tool, and observe whether your mineral can be scratched or not. For example, if it can be scratched by the glass, it means the mineral is softer than 5.
You are welcome to follow the next step-by-step instructions, created by a professional geologist, with insights obtained during the fieldwork.
Step 1: Clean the stone
The first step is always a sample preparation stage. Clean your mineral sample with water and a brush to remove dust or dirt. Otherwise, you can test the hardness of dirt particles, which can contain very hard debris of quartz sand, for example.
Step 2: Observe your sample
Observe your clean stone. Pay attention to what kind of sample you have. Is it a crystal of mineral, or is it a rock (a naturally occurring aggregate of minerals)? A scratch test is suitable for minerals or, occasionally, for coarse-grained rocks only.
Also, check the mineral for weathered surfaces. They usually look dull, reddish, orange, and earthy. These zones should not be used for the scratch test, as the mineral there is already disintegrated.
Important note! The mineral for a scratch test should be big enough and fresh (without weathered surfaces).
Step 3: Protection
Be careful. Use gloves to protect your hand and something resistible, like a rubber mat or wooden board, to protect a tabletop or a floor.
TIP: Gloves will give your fingers valuable protection when scratch testing your rocks. Check out the best gloves for rockhounding in the article below:
3 Best Gloves for Rockhounding: Protect Your Hands
Step 4: Choosing a strategy
You can have two different approaches to the scratching test. The first one is to use index minerals from the Mohs scale. The second – is to use tools (described in step 6) with an equivalent hardness to the Mohs scale.
Step 5: Scratching with index minerals from the Mohs scale
In case you have the Mohs Mineral Hardness Test Kit with 9 minerals, start from the softest one. Scratch your mineral sample with talc and vice versa. Continue to increase hardness and test your mineral with gypsum, calcite, fluorite, apatite.
Hold your test sample firmly and make just one intense scratch. It is not a rubbing test. Just one qualitative scratch line will be enough to understand what mineral is harder. Scratching is a comparison. You just need to find an index mineral, which is harder than your test sample.
The Mohs Hardness Scale:
|Talc||1 (the softest)|
|Feldspar (sometimes indicated as Orthoclase)||6|
|Corundum (ruby and sapphire)||9|
|Diamond||10 (the hardest)|
Step 6: Scratching with equivalent hardness tool
The general approach is similar here. At first, try to scratch your mineral sample with a fingernail, which hardness is equal to 2 – 2.5 on the Mohs scale. Continue to test your mineral with a copper wire that is 3 3.5 on the Mohs scale.
After that, try to scratch a piece of glass (5 – 5.5) with your mineral. If your mineral is still harder, you can use a steel file (6 – 6.5). In case the mineral is still harder than a steel file, you can use a quartz crystal (7).
In case your mineral scratches quartz crystal, and at the same time quartz cannot scratch your mineral back, it means you have a really hard one. Most probably, it is a gemstone. One of the gemstone characteristics is durability, so most gemstones have a hardness of more than 7.
Tabe of tools with an equivalent hardness to the Mohs scale
|Fingernail||2 – 2.5|
|Cooper wire or coin||3|
|A piece of glass||5 – 5.5|
|Knife blade||5 – 5.5|
|Steel file||6 – 6.5|
Step 7: Double-check the results
All test results should be double-checked to be reliable. If you have found a mineral or a tool that is harder than your sample, try to scratch it back.
Sometimes a chalky line from a destroyed softer mineral can be perceived as a scratch. This is misleading. So it is better to make a second try and to make a scratch test vice versa.
Step 8: Compare results to the Mohs scale
When you have found the tool or mineral which is harder than your mineral and double-checked that the previous softer mineral or tool is definitely softer than your mineral, you can determine a span of hardness. For example, your test mineral is harder than glass (5 – 5.5) but softer than a steel file (6 – 6.5), which means your unknown mineral hardness is around 6.
Check the Mohs hardness scale and mineral encyclopedias. Most probably, your tested mineral is feldspar.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
TIP: Have you ever heard of double refraction of crystals? Double refraction is interesting optical property of crystals. Find out more in the article below:
8 Minerals That Show Double Refraction (+ Explanation Why)
Some Useful Insights for the Scratch Test
- It will save you some time if you start with a piece of glass or a knife. These tools’ hardness is usually around 5 – 5.5, so beginning a scratch test with them will separate the Mohs scale into two groups: harder than 5 minerals and softer than 5 minerals.
- Use a combination of napkin and magnifying glass to check whether it is really a scratch or just powder left by a softer mineral. Wipe a line and check if there is a scratch or not to be sure of the result.
- Equivalent tools are not so precise for hardness identification. You should always keep in mind that glass and knife blade hardness can vary because of different compositions of materials. For the precise scratch test, we recommend using a Hardness Pick Set – a set of calibrated pencil-looking picks composed of alloys carefully selected to match the hardness of the Mohs index minerals.
What Minerals Would You use in a Scratch Test?
All 10 index minerals from the Mohs scale can be used in a scratch test: talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum (ruby and sapphire), and diamond. However, a diamond is not necessary. It is the hardest, so it will scratch any mineral anyway.
Can rocks scratch glass?
Roks which are harder than glass (or harder than 5.5 on the Mohs scale) can easily scratch it. Such minerals as feldspar, quartz (amethyst, citrine), tourmalines (rubellite, indicolite), beryls (emerald, aquamarine), topaz, corundum (ruby and sapphire), and diamond can scratch glass.
A scratch test is used to determine the relative hardness of minerals and to identify a stone according to the Mohs scale of relative hardness. A scratch test is one of the most important tests for mineral identification because of the next advantages of this method.
- A scratch test is really simple and fast, so it can be performed even at home.
- You don’t need time-consuming preparation and comprehensive equipment for the scratch test.
- Tools for the test are easily accessible at home (fingernail, cooper coin or vire, a piece of glass, a knife, a steel file).
A scratch test can be performed by simply following the next 8 steps:
- Clean the unknown stone before the test.
- Carefully observe it and find the best part for the test.
- Take care of your hands and surrounding protection.
- Choose a strategy based on the tolls and minerals available for the scratch test.
- Do a scratch test by scratching your unknown mineral by index mineral from the Mohs scale one by one.
- Do a scratch test by tools with equivalent hardness (fingernail – 2, copper – 3, a piece of glass, and a knife – 5 – 5.5, a steel file – 6 – 6.5).
- Double-check your results by reverse scratching.
- Compare the result with the Mohs scale and find the probable mineral.
TIP: Another option to test for hardness of your rocks is using a Mohs scale test kit. Check out the best mohs scale test kits on the market in the article below:
3 Best Mohs Scale Test Kits: Test Hardness of Your Gemstones