What is a Streak Test and How Do You Perform One?

Streak testing is a great tool for helping you to identify that cool rock you found. This blog covers what a streak test is and how you can perform one.

<This article contains Amazon links but will also give you free ways of doing things, please don’t be mad!>

...and sodalite, which has a white to a light blue streak.

A Streak Test can easily tell the difference between blue quartz (colourless streak)

Mineral identification is not easy. Not only are several minerals the same colour or shape, but any AI attempt to identify your rock will be wrong 98% of the time. Colour simply isn’t enough to id with and that’s all a computer has to go on. Streak Testing is therefor vital for mineral ID - whether you use an app, an AI program, or not.

In fact, there are a bunch of easy tricks you can learn to help you identify rocks on your own, with just a little bit of reading. By the time you read the next section, you will know what a streak test is. Give me one more section of your time and I will tell you how to perform it yourself, too.

What is a Streak Test in Mineral Identification?

A streak test in geology does not refer to a naked scientist running across a football field. A streak test involves making a mark with a mineral (exactly as if it were the graphite in your pencil). The mark – or streak – is then examined for colour. The colour that the streak is will narrow down your list of possible minerals.

When coupled with other rock ID methods like Mohs hardness testing and examining the location information where you found your rock, performing a streak test could give you the answer you are looking for.

Essentially, by grinding a little of the sample rock specimen on a streak testing plate, you are able to break it down into smaller particles which give an indication of the true colour and hardness of the rock. But how do you perform a streak test if not all rocks can write?

How Do You Perform a Mineral Streak Test for Rocks?

To perform a streak test, simply take the mineral you want to identify and draw a line with it. The problem is that not every stone draws a line, and not every surface can be drawn on. Therefor, to perform an accurate streak test, you will need some unglazed porcelain or a streak testing pad, as detailed below.

Here are a few good examples of common streak test results which you might expect to get.

A Streak Test for Gold

Gold will always streak golden. The streak test results for fool’s gold, though, will be greenish black instead. Gold is always golden unless it is wrapped in iron. Even iron wrapped gold will streak golden.

A Streak Test for Quartz

Quartz is interesting because common rock crystal has no streak. This means that all the minerals that come under the quartz banner also don’t leave a skid mark. Just for extra clarification, that means minerals like citrine, jasper, chalcedony, agate, amethyst, rose quartz, smoky quartz, and a few others. A diamond will also leave no streak, just to confuse you.

To check what your streak mark means, you really ought to use a streak test chart.

Streak Test Chart

A streak test chart tells you which minerals have which streak colours. It is always worth downloading one and saving it to your phone or computer. Geology.com have a streak test chart which is unmatched. We are not sure if we are allowed to share it, but the link will take you to it.

Here are some of the most common minerals and their streak colours to save you a potential trip:

• Apatite, Barite, Calcite, Dolomite, Fluorite, Halite, Gypsum, Prehnite, Orthoclase, Plagioclase, Rhodochrosite, Rhodonite, Serpentine and Zoisite all have a white streak

  
  

• Quartz is colourless, as are Topaz, Tourmaline, Zircon, Garnet, Nephrite Jade, Jadeite, Beryl, Corundum, and Diamond.

  
  

• Silver is a very silvery, metallic white. Copper is a bright, coppery red.

  
  

• Arsenopyrite, Bornite, Chalcocite, Galena, Marcasite, Molybdenite, Pyrrhotite, and Uraninite all come in a greyish black to black colour.

  
  

• Sodalite and Azurite might streak light blue, which is refreshing.

  
  

• Talc, Malachite, Diopside, and pyrite can all have a greenish tint to the streak.

Jade, Nephrite Jade, and Jadeite, are three of the minerals which steak colourless, like quartz does.

What Equipment Do You Need for a Streak Test?

There are two easy ways to get an accurate streak test reading. You will need the mineral in question, a streak test chart such as the one above, and a piece of unglazed porcelain. You can usually find this on the underside of the lid to your toilet cistern. You might also find it on the bottom of a mug.

If you do not have any unglazed porcelain to hand, or if you want to perform streak tests without carrying your toilet with you, then you might want to buy a streak testing kit. You can find streak testing kits on Amazon. There are several.

If you want them for jewellery and gemstone identification, this set of four looks good. If you want to combine streak testing with Mohs hardness testing in one easy kit, then this one is brilliant. If you just want to buy the unglazed porcelain, then you can do that too. This set comes with 12 streak plates you can use.

While we were there, this 7pc mineral ID kit caught our eye because it includes objects of known hardness for Mohs testing, a jeweller’s loupe for magnification, and two streak test pads.

Of course, I have been known to use another rock in a pinch, particularly slate. But if you want an accurate reading, it has to be the testing kits or the unglazed porcelain.

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