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3.3 Mineral Luster, Transparency, Color, and Streak

Luster & Transparency

How does a mineral reflect light? Luster is the appearance of a fresh mineral surface when reflecting light. It has nothing to do with the color of the mineral, but the scattering of light off the mineral surface. Here is a visual guide to the different types of mineral lusters! [1]

LUSTER TYPE EXAMPLE
Adamantine
Diamond with Adamantine Luster
Figure 3.3.1 Polished diamond
Vitreous
Quartz demonstrating vitreous luster
Figure 3.3.2 Vitreous quartz
Silky
Selenite Gypsum demonstrating silky luster
Figure 3.3.3 Selenite with silky luster
Greasy
Greasy luster demonstrated by Graphite
Figure 3.3.4 Greasy graphite
Waxy
Waxy luster shown by Mimetite
Figure 3.3.5 Mimetite with waxy luster
Dull/ Earthy
Earthy, Dull luster as shown by Kaolinite
Figure 3.3.6 Kaolinite with dull luster.
Metallic
Metallic luster as shown by pyrite
Figure. 3.3.7. Pyrite with metallic luster.

Color

Sometimes color can be an identifying property of minerals.

For instance, malachite is always green, azurite is always blue, and purpurite is always purple.

Green Malachite and Blue Azurite
Figure 3.3.8 Rock containing green malachite and blue azurite.

Other times, minerals have trace elements in them that make them a wide range of different colors.

All of the minerals in the image above are quartz (SiO2). However, they range in color from clear/white, purple, pink, gray, and amber. [1]

 

The many different colors of quartz stones
Figure 3.3.9 Quartz can be nearly any color as shown in these pebbles.

Streak

Another visual property of a mineral is its streak. Streak is the color of a mineral in powdered form. When you scratch a mineral across a porcelain streak plate, what color does it leave behind? This color can be distinguishing!

The chart below summarizes how we can generally identify a mineral’s streak on a testing plate.

Mineral Name Bulk Color Streak Color
Azurite Dark to light blue Baby blue
Hematite Red to Dark Gray Rust red
Pyrite Brassy Yellow Dark Gray
Graphite Dark Gray Dark Gray to Black (Pencil Lead)
Realgar Red to Orange Orange

In the exercise below, you can determine the color of a mineral’s streak, if it has any, in the identification window that opens up by dragging your cursor across the white streak plate with the rainbow outline. If a color shows up, determine which mineral(s) exhibit that streak color and bulk color in the table above.

Streak Plate

Sample 1: https://wwwi.scottsdalecc.edu/otter2/explorer.html?tag=0793dfb1

Sample 2: https://wwwi.scottsdalecc.edu/otter2/explorer.html?tag=8b286b35

Sample 3: https://wwwi.scottsdalecc.edu/otter2/explorer.html?tag=f4b03b4d

Sample 4: https://wwwi.scottsdalecc.edu/otter2/explorer.html?tag=28bcda53

One more useful trick! The streak of a mineral can save you time and money! Pyrite (FeS2) is well-known as fool’s gold because of its metallic-yellow color. If you’re in a pinch, and can’t tell pyrite apart from genuine gold, consider a streak test!

Brassy yellow pyrite leaves a dark gray streak and pink rhodochrosite leaves a white streak.
Figure 3.3.10 Some minerals leave a different streak color than their bulk mineral would indicate. Left: Pyrite, Right: Rhodochrosite.

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