Luster and Light

Image credit: James St. John. https://commons.wikimedia.org/wiki/File:Gold_nugget_(placer_gold)_(Pennsylvania_Mountain,_Alma_Mining_District,_Park_County,_Colorado,_USA)_4_(16856591407).jpg 

The Bottom Line on Luster - Here on Top!

In an introductory course, luster is a described as a property of light reflection that separates metallic from non-metallic minerals.

Determining luster can be difficult for a beginner. A dark colored sample of weathered magnetite (metallic luster) might be mistaken for an earthy sample of hematite (non-metallic luster), but these two will differ in other properties, for example magnetism.  Other physical properties are more easily and definitively determined, such as electrical conductance, hardness, streak, reaction to acid, magnetism, etc.  leading to fewer errors and a faster route to the identification of a mineral. Check the Mineral Bank for this and additional information.  

Bottom Line: I created a  replacement activity for this traditional observation of luster - the flashlight circuit test. My students find it fast and fun. Results are almost always unequivocal. And all that's needed are $1 flashlights. Check out the section on electrical conductance for details. 

Errors/Suggestions: Contact Scott Brande (see footer).

What is Luster?

Background. Luster is a property of the mineral response to light. When light illuminates any matter, you may see see one of several different responses.

• light may reflect off the surface, like from a mirror

• light may reflect off the surface and be partially absorbed (penetrate) into (and perhaps through) the mineral

Although the appearance of a mineral may vary with conditions of illumination, we will simplify our interpretation into two broad categories - metallic and non-metallic luster.

• Metallic luster means reflected light resembles a polished metal surface. 

• Otherwise, the appearance of reflected light is termed non-metallic  and this appearance may vary. Terms used for non-metallic luster include glassy (or "vitreous"), pearly (like mother-of-pearl), etc. For more details about the different terms given for luster, consult this link -  luster in mineral identification.

Learn Luster in Single Crystals

Learn Luster in Rocks

A mineral, by definition, is a solid chemical compound with ions and molecules arranged in long-distance, repetitive, geometric order. The above examples show single crystals of significant size for handling and observation. 

The determination of luster becomes more difficult and uncertain as crystal size becomes smaller, eventually microscopic. That's why you'll need to examine a sample to determine multiple properties that help you to narrow down a list of possible names for identification. Don't spend too much time on evaluating luster of rock samples. Make sure you observe other properties (e.g., hardness, streak, cleavage, etc.)

Here are some examples of luster in rocks (multi-crystalline aggregates) where crystals approach microscopic in size, making luster more difficult to evaluate.

Demonstration of Luster

Watch the video demonstrations below.

Instructions: The Luster Test

Caution - learning to identify the particular type of luster takes practice, and for the beginner, may be difficult. Other properties are more easily determined with less potential error, and lead more directly to the identification of the mineral. So we downgrade the importance of luster as a diagnostic property. For example, if one misidentifies a mineral as having a non-metallic luster when it actually exhibits a metallic luster, one would then discover with other properties that the combination of other properties does not match minerals in the non-metallic group. A wrong determination of luster will be apparent, thus sending one to the other category for further assessment.

Materials for test

• Mineral sample

• Overhead light

Procedure for the reflection test

• Hold the sample to orient it with any planar (flatter) surfaces facing an overhead light. 

• Rotate the sample back-and-forth to reflect light from a source. 

• Observe the reflection of light from any flat surfaces.

Possible test results and interpretation

• Metallic. As the mineral is turned against the overhead light, we observe a flash described as that from a polished metal surface. We identify this type of reflection as metallic.

• Non-metallic. As the mineral is turned against the overhead light, we either observe a flash that does not resemble polished metal. We identify this type of reflection as non-metallic. For our quick-and-dirty assessment of luster at this time, we don't need to determine the type of non-metallic reflection (e.g., glassy, pearly, earthy, etc.).

Procedure for transmission test

• Hold the mineral in front of a light (the brighter, the better).

• Orient the sample and look through the thinnest edge or corner.

• Observe any light passing through the mineral.

Possible test results and interpretation

• Non-metallic luster. If the sample absorbs and transmits any light, indicated by a glow, (e.g. best to find a thin edge), then the mineral is non-metallic.

• Metallic luster.  If the sample reflects all light in a mirror-like (not glassy) fashion, then mineral may be (but might not be) metallic. Extreme caution about your conclusion is indicated here.