Streak
Definition
The color of a mineral in powdered form, typically observed by rubbing the mineral on an unglazed porcelain plate.
Example
Hematite appears black or silver but has a distinctive red-brown streak.
The Power of the Streak Test
Streak is the color of a mineral in its powdered form. While the external color of a mineral can vary wildly due to impurities (allo-chromatic), the streak color is remarkably consistent (idio-chromatic). For example, Hematite can be silver, black, or rusty red on the outside, but its streak is <strong>always</strong> a reddish-brown.
How to Perform a Streak Test
To find the streak, rub the mineral firmly across an unglazed porcelain tile (streak plate). The plate has a hardness of about 6.5. Minerals harder than 6.5 (like Quartz or Diamond) will not leave a colored streak; they will scratch the plate instead (leaving a white powder which is actually the plate itself). Therefore, the streak test is most useful for identifying softer, metallic, or earthy minerals like oxides and sulfides.
Diagnostic Examples
- Gold: Yellow streak (distinguishes from Pyrite).
- Pyrite: Greenish-black streak.
- Hematite: Red-brown streak.
- Galena: Gray-black streak.