To perform the streak test, you rub a specimen on a piece of unglazed porcelain tile. Go to your local hardware store, and look at the backs of their floor tiles. You want a tile that is made of porcelain, and the unglazed back should have a white color.
These tiles are very hard (hardness 6.5), and they have a rough surface. As you rub the mineral across that surface, it acts like a file, powdering up some of the mineral. The color of that powdered mineral is its streak color.
At first, the streak test can be puzzling. You would expect that if you rub a yellow mineral on your streak plate, it would leave a yellow streak. After all, you are just powdering up a bit of the specimen. Why would small pieces look any different from large pieces?
How the streak test works
When you powder a mineral, you change its luster to an earthy luster. It may start with a luster that is metallic, vitreous, or dull, but its streak will always have an earthy luster. Some lusters, especially the metallic lusters, can hide the true color of the mineral. By removing that metallic luster, you often see a very different color. A good example of that is the mineral hematite. Hematite is iron oxide, chemically very similar to rust. But, if your piece of hematite has a metallic luster, it will be a very shiny, dark gray color. Powdering some of the hematite with the steak test will show its real color, a red streak the same color as rust.
The streak test also hides many of the colors caused by impurities. A good example is the mineral calcite. Pure calcite is clear, but with impurities, it can be yellow, brown, green, or almost any other color. Powdering the mineral hides that color, so calcite always has a white streak, no matter what color impurities have colored the specimen.
Not all minerals have a streak
A porcelain tile has a hardness of about 6.5. Any mineral that has a hardness of 6.5 or less will leave a streak, but what happens if you try to perform the streak test with a mineral that is harder than 6.5? Nothing. You don't get any streak. In fact, if the mineral is much harder, it may scratch the tile.
At first glance, it can be difficult to tell if a mineral has a white streak or no streak. They may both look the same on the white porcelain, but if you rub your finger across the tile, and then look at it, a white streak will show up as white powder on your finger. If there is no streak, your finger will be clean.
Tips on Testing
- Be careful testing the streak of delicate or prized specimens. As with the hardness test, the streak test can damage these specimens.
- Look closely at your specimen. Many specimens contain more than one mineral. Be sure you are testing the mineral you want to identify.
- Always start with a clean spot on your streak plate or tile. White powder from a previous test can give you a false result for a mineral with no streak.
- When your streak plate gets dirty, you can clean it with cleaning products meant for porcelain sinks. Comet and Soft Scrub work very well.