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Minerals’ streak is one of the most important tests for mineral identification, especially while working with native metals and sulfide minerals. Mineral’s streak test is underestimated and sometimes missed during an identification process. However, today we will tell you in what cases and with what kinds of minerals this test is inevitable.
A mineral’s streak is the color of mineral powder when it is rubbed on a square of unglazed porcelain called a streak plate. Minerals’ streak is an important test for mineral identification as sometimes mineral species can occur in a rainbow of color, but the streak color is always the same.
Gold and fool’s gold (pyrite) are very similar-looking minerals. Do you know that the streak test can differentiate them in a flash? It’s just one small example. We have something more to impress you. Further, we will discuss what is needed to conduct a streak test and what minerals are the most suitable for identification.
If you are interested in checking out a simple but useful mineral streak test kit you can find it by clicking here (Amazon link).
What Does Mineral Streak Mean?
A mineral streak means the real color of a mineral. Sometimes minerals can be colored by impurities, or the color can be a result of crystal structure defects, or minerals can be phenomenal (like spectrolite or opal). Mineral streak always shows the same color for the one mineral species.
Various colors of mineral streak allow differentiating between similar-looking hematite and magnetite; or between gold and pyrite (fool’s gold).
However, the streak test has some limits:
- Minerals with non-metallic luster. These minerals usually have a white streak or a very light-colored streak that is not consistent from one sample to the next.
- Silicate minerals, like quartz, garnets, beryls, tourmalines, and many others, are usually harder than the streak plate (which is about 6.5 on the Mohs Hardness Scale) and thus can not be powdered and do not produce streaks.
Important note! Minerals that are harder than streak plates can produce a tricky chalky-white streak. This streak is not a powdered mineral – it is a scratched streak plate. Try to remove the powder and observe if there is a scratch on the streak plate. Minerals that are harder than the streak plate are said to have “no streak” or a “colorless streak.”
What Does a Mineral’s Streak Tell You?
The Mineral’s streak tells the real color of the mineral. Sometimes mineral seems to be black; however, the streak color will be red, like in the case of hematite. Comparing the streak color to the tables tells you that only hematite has this pair of colors. So mineral identification took you only a few seconds.
The color of a particular mineral may vary, but the streak is generally constant. The streak may be the same color as the mineral or an entirely different color but the streak of all-white minerals, including calcite, is white.
What Is a Mineral Streak Test?
A mineral streak test is one of the tests needed for mineral identification. Streak is a fine powder of the mineral produced when it is scratched on a streak plate. Sometimes the color of the streak is different than the color of the non-powdered mineral. The colors will help you to identify the mineral.
A mineral streak test is one of the mineral identification methods. This method is not suitable for all kinds of minerals, but for about 20% of minerals – a streak test is the best choice.
It is better to conduct a streak test after the Mohs hardness test, as it will save you some time if the established hardness is more than 7. A streak plate is made of porcelain and is barely harder than 6.5 on the Mohs scale.
That is why such minerals as quartz, topaz, corundum (ruby and sapphite), diamond, all types of garnets, beryls, and tourmalines are not suitable for a streak test as they are harder than a streak plate and will simply scratch it.
TIP: Rock color is the first property we pay attention to. Rocks occur in a branch of colors, hues, and saturation. Find out everything you need to know about rock’s color in the article below:
Rock Colors: What Determines Color & Why Different Colors
How Do You Determine a Mineral’s Streak?
To determine a mineral’s streak, you need to rub a mineral against an unglazed porcelain plate (streak plate). The streak plate can be white or black.
Observe the color of the streak and compare it to your mineral color. Avoid doing streak tests with fine-grained rocks and a weathered surface of minerals.
To determine a mineral’s streak correctly, it’s necessary to make a short sample preparation:
- First of all, it is necessary to clean the mineral. Any clay particles can distort the streak color.
- After that, keep an eye on weathered surfaces. They are mostly composed of oxides which will give absolutely different streak colors.
Important note! The best option for any mineral identification test, including the streak test, is to have a fresh mineral surface.
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)
How Do You Test the Streak of a Mineral?
To test the streak of a mineral, rub your clean sample against an unglazed porcelain plate called a streak plate. Minerals that are softer than porcelain (6 – 6.5 on the Mohs scale) will leave a colorful streak. Observe the color and compare it with your mineral and mineral characteristics table.
Here is a step-by-step instruction on how to test a streak of a mineral.
- Prepare a streak plate. It is generally a piece of unglazed porcelain. It can be the reverse side of any tile. (Learn more about tools you need in the next chapter).
- Clean your mineral sample. It should be free from a weathered surface. The best place for the test is a fresh fracture.
- Hold a streak plate firmly.
- Take a mineral sample into the hand you usually write.
- Draw one confident line by a mineral with a little pressure applied.
- Observe a streak. If it’s too opaque, smudge it with your fingertip.
- The test is done! Now you can compare the color of the streak to the initial mineral color. If it differs, you have a great chance to identify the mineral quickly just by checking the table prepared for you in the last section.
- The last important step is to clean a streak plate. Wipe it with a wet paper napkin to remove mineral powder and polish the plate with fine-grained sandpaper.
TIP: The mineral’s hardness is one of the most helpful characteristics in mineral identification. Check out the complete guide on testing mineral’s hardness in the article below:
DIY Guide: Testing Mineral’s Hardness (Explained by Expert)
What Tool is Used to Determine a Mineral’s Streak?
Streak plates (pieces of unglazed porcelain) are used to determine a mineral’s streak. Streak plates of white and black colors are offered in mineral identification kits. The backside of the kitchen or bathroom tile is a good streak plate alternative. Sometimes a steel file can be used as a streak plate.
For a successful streak test, you will need the next tool:
- A streak plate (it can be a piece of unglazed porcelain or a reverse side of kitchen or bathroom tile). The streak plate can be either black or white. Depending on the color of the mineral you are testing. Dark minerals’ streaks are better observed on the white plate and vice versa.
- Sandpaper. Sandpaper is needed to clean the streak plate and to remove the colorful mineral powder. Gentle polishing allows you to make your streak plate reusable.
- Magnifying glass. This tool is not necessary but may be helpful in the case of hard minerals. A magnifying glass will help to check whether it is a white streak or a scratch on the streak plate.
TIP: If you are interested in checking out a simple but useful mineral streak test kit you can find it by clicking here (Amazon link).
What is the Difference Between a Mineral’s Streak and Color?
The main difference between a mineral’s streak and the color is that streak is a color of a mineral in a powdered state. The color of the streak is considered to be the real color of the mineral as it eliminates the influence of impurities or structural defects, which create the color of the crystal.
Some minerals leave a streak similar to their natural colors, such as malachite, lazurite, and native gold. Other minerals leave surprising colors.
For example, calcite, which always has a white streak, can appear in a wide range of colors, including white, yellow, orange, brown, pink, blue, lavender, gray, green, and black.
Calcite color is created by traces of other elements or physical impurities. Despite it, the streak color will be also white. That is why the color of the streak is considered to be a real color of the mineral.
Another example of why the streak color is important – is that minerals can sometimes have phenomenal optical effects, which hide the real color of minerals. Such minerals are labradorite or spectrolite, opal, sunstone, or aventurine.
Further, we are listing minerals that are the most suitable for the streak test:
Fluorite (calcium fluoride, CaF2) occurs in any possible color and color combinations. It can be vibrant blue, green, purple, red, orange, or colorless.
Fluorite’s relative hardness is 4 on the Mohs scale, making it an ideal candidate for the streak test. All specimens of fluorite of any color have a white streak.
TIP: Fluorite is one of the most widespread minerals in crystal collections. Check out the differences between real and fake fluorite in the article below:
Real vs. Fake Fluorite: Focus on These 5 Differences
Specimens of hematite (iron oxide, Fe2O3) can be black, red, brown, or silver and occur in a wide variety of habits.
However, all specimens of hematite produce a streak with a reddish color. A red streak accounts for its name, originating from the Greek and meaning blood.
This unique property of hematite can be used to differentiate hematite from a large number of other metallic luster minerals with high specific gravity, similar color, and crystal habits (magnetite or galena).
Galena (lead(II) sulfide, PbS), which can be similar in appearance to hematite, is easily distinguished by its gray streak.
Pyrite (iron sulfide – FeS2 ) always has a brassy yellow color. It is sometimes called a fool’s gold because people mistook specks in ores for real gold. A streak test can differentiate between gold and pyrite at a glance. All specimens of pyrite produce a black streak, but native gold has a gold streak.
Here is a general Table of the selected minerals, which can be identified with the help of the streak test. For your convenience, minerals are separated into two groups: yellowish and grayish. Please, be informed that the colors are distorted to make the table legible. The general idea of hues is preserved.
|Yellow||Pale to golden-yellow||Gold||Au|
|Black to greenish||Pale-brass||Pyrite||FeS2|
|Gray to black||Bronze tarnishes to dark blue and purple||Bournite||Cu5FeS4|
|Yellow-brown||Yellow-brown to black||Limonite||FeO(OH)·nH2O|
|Chocolate-brown||Black to dark-brown||Chromite||FeCr2O4|
|Light-gray to silver||Silvery-white tarnishes to black||Silver||Ag|
TIP: A scratch test is an essential method of mineral identification. It is widely used because of its simplicity. Check out the complete guide on performing a scratch test on rocks in the article below:
Performing Scratch Test on Rocks (Follow These 8 Steps)
A mineral’s streak is the color of a mineral in a powered state after rubbing the mineral against a strake plate (a piece of unglazed porcelain). A streak test is one of the most important mineral identification tests, especially for native metals and sulfide minerals.
Streak reveals the real color of the mineral. The same mineral species can occur in a range of colors, like calcite or fluorite. However, the color of the streak will be always the same – white.
Therefore, the color of the mineral and the color of the streak are not always the same. Some mineral colors are created due to impurities or structural defects.
However, these characteristics don’t influence the streak color. That is why the streak test allows identifying the mineral or differentiation between similar-looking species.
The streak test helps differentiate between:
- gold and pyrite (fool’s gold);
- hematite and magnetite;
- hematite and galena;
- chromite and magnetite;
- cooper and chalcopyrite.
The streak test also has some limits. It cannot be applied to minerals whose hardness is more than 7 (for example, quartz, topaz, corundum (ruby and sapphite), diamond, all types of garnets, beryls, and tourmalines), as a streak plate is softer and will be simply scratched.
TIP: A Mohs hardness test is one of the most useful tricks that should be in all rockhounds’ playbooks. Check out the best Mohs hardness test kits in the article below:
3 Best Mohs Scale Test Kits: Test Hardness of Your Gemstones