A mineral's streak is the color it has when ground to a powder. Some minerals that occur in a range of colors always have the same streak, thus streak is considered a more stable indicator. The great majority of minerals have a white streak, but there are a few well-known minerals for which the streak is an important property.
The simplest way to make a powder is to grind the mineral on a small rectangular piece of unglazed ceramic called a streak plate. Streak plates have a Mohs hardness around 7, but be sure to check your streak plate against a piece of quartz (hardness 7) because some are softer and some harder. The streak plates shown here have a hardness of 7.5. An old kitchen tile or even a sidewalk can also serve as a streak plate. Mineral streaks can usually be wiped off easily with a fingertip.
Images 1-8 of 8
- Graphic Index
- Text Index