Streak
Image credit: Scott Brande
The Bottom Line on Streak - Here on Top!
Streak refers to the color of a sample crushed to microscopic particles. A few minerals exhibit about the same color in a bulk (larger) sample compared to crushed fragments and powder. Check the Mineral Bank for this information. For example, the mineral sulfur typically is yellow-ish in bulk, and its crushed powder tends also to be yellow-ish. Other minerals exhibit a streak that differs in color from the bulk sample. For example, note the green crystal above that differs in color from its white crushed powder.
Bottom Line: The color of a mineral's streak will not, by itself, be diagnostic of a particular common mineral. When a sample's streak differs from the bulk color, it is an important property when used in combination with others to identify a mineral by name.
Errors/Suggestions: Contact Scott Brande (see footer).
"Bulk" Sample Color
Background. A single mineral may exhibit different colors due to, for example, chemistry (e.g., trace amounts of included metal ions), or physics (e.g., diffraction of light; ionizing radiation).
Here's an example of color variation in gem quality garnets. https://geogallery.si.edu/10002799/grossular-garnet
Bulk Sample and Streak Color
Watch the video below.
What's Streak?
A powder produced by crushing a larger fragment is composed of microscopic particles.
The property streak refers to the color of the powdered crush.
The streak test is done by actually crushing a thin edge or corner of a bulk sample against a porcelain plate, which has a relative hardness on the Mohs scale of about 7.
Image credit: Scott Brande
Why, and When, is Streak Important?
A mineral's color may be caused by many different factors of physics and chemistry, the details of which are not important in the context of using streak in mineral identification. What is important follows.
White streak
Many common minerals of different colors produce a white streak when crushed and powdered.
Gray to black streak
The crushed powder of a number of common minerals appears lighter to darker gray, or even black.
Colored streak
The most interesting and important result of a streak test is when the color of the powdered sample differs in color from the color of the bulk specimen. This happens in only a few of the common minerals. An example is pyrite, which typically is brassy-yellow, but the streak is black.
How Do We Name Colors?
A vocabulary for color and grayscale
The determination of the color of a mineral sample would best be done by instrumental measurement, such as a spectrometer. For our purpose, a quick-and-dirty assignment of color will be done by eye and a simplified vocabulary that does not do justice to the full spectrum of visible light which varies continuously in frequency/wavelength. You may recall the acronym for the colors of a rainbow - ROYGBIV - red/orange/yellow/green/blue/indigo/violet. We'll simplify this seven color spectrum even more by dropping indigo.
A grayscale varies continuously from white to black. We'll make judgments by eye when assigning one of three terms on this scale: white / lighter to darker gray / black
A simplified color / grayscale spectrum
Streak Examples
Streak same color as bulk sample
yellow-ish streak, yellow-ish bulk
Image credit: Scott Brande
red-ish streak, red-ish bulk
Image credit: Scott Brande
Streak color different from bulk sample
white streak, green-ish bulk
Image credit: Scott Brande
darker gray (to black) streak, yellow-ish (brassy) bulk
Image credit: Scott Brande
Instructions: The Streak Test
Materials Needed
sample large enough to be gripped securely in hand
streak plate
Streak plate
A streak plate is made of fused silica.
The streak plate surface is a bit bumpy.
A streak plate has a hardness of about 7 on the Mohs scale.
Caution
Assume the sample's hardness is in fact less than that of a streak plate, about 7 on the Mohs scale. Performing a streak test correctly requires enough vertical force exerted on the sample to crush a sharp corner or edge against the streak plate to produce a powder. If insufficient force is exerted, one might incorrectly conclude that the sample is harder than a streak plate (about 7 on the Mohs scale).
Materials for the streak test
mineral sample
streak plate
Procedure for the test
Hold the mineral in one hand to expose a sharp corner or edge.
Steady the streak plate on a table with your other hand.
Press the sample against the streak plate.
Drag the sample across the streak plate with a short, sharp stroke.
Observe the surface of the streak plate.
Possible test results and interpretation
The sample either crushes and leaves a line of powder on the streak plate, or it does not.
Mineral crushes and leaves a line of powder against the streak plate. The sample has a Mohs hardness <7 because it crushes against the streak plate. Observe the color of the sample powder. Compare the streak color to those in a mineral identification chart.
Mineral does not crush against the plate - no powder produced. Mineral streak is not applicable. Mineral Mohs hardness is greater than (>) 7 or about equal to (=) 7 because mineral does not crush against the streak plate.