Streak (mineralogy) – Harry & Co Jewellery

Definition and Importance of Streak

Streak is the color of the powder produced when a mineral is dragged across an un-weathered surface.
It is a diagnostic tool in mineral identification.
Streak is particularly important for opaque and colored materials.
It is less useful for silicate minerals.
Minerals with no visible streak are considered to have a white or colorless streak.

The apparent color of a mineral can vary due to trace impurities or disturbed crystal structure.
Small impurities do not greatly affect the absorption of light when a mineral is crushed into powder.
The color of the powder produced during streak testing is more consistent than the apparent color.
Impurities can radically change the wavelengths of light reflected by the mineral.
Streak testing breaks the specimen into randomly oriented microscopic crystals.

The surface across which the mineral is dragged is called a streak plate.
Streak plates are generally made of unglazed porcelain tile.
The unglazed underside of a porcelain bowl or vase can also be used as a streak plate.
The back of a glazed tile can serve as an alternative streak surface.
Sometimes comparing the streak with another streak plate can provide a more accurate description.

A streak can only be made by minerals softer than the streak plate.
The Mohs scale of mineral hardness is used to determine if a mineral can produce a streak.
Minerals with a hardness of around 7 on the Mohs scale can create a streak.
Harder minerals require filing or crushing to determine the color of the powder.
Most harder minerals have an unhelpful white streak.

Some minerals leave a streak similar to their natural color, such as cinnabar, lazurite, and native gold.
Fluorite always has a white streak despite appearing in various crystal colors.
Hematite, with a black appearance, leaves a red streak, giving it its name derived from the Greek word for blood.
Galena, resembling hematite, can be distinguished by its gray streak.
Streak colors can provide additional identification clues for specific minerals.

Reference	URL
Glossary	https://harryandcojewellery.com.au/blogs/glossary/streak-mineralogy
Wikipedia	http://en.wikipedia.org/wiki/Streak_(mineralogy)
Wikidata	https://www.wikidata.org/wiki/Q107775
Knowledge Graph	https://www.google.com/search?kgmid=/m/02q3w0d