What is Mineral Luster?

When my daughter was in first grade, she had an in-class field trip where someone came to their classroom to teach them about rocks and minerals. Intrigued, I asked if I could sit in on the presentation. I thought the presenter did a great job of teaching six-year-olds some basic properties of minerals until she got to the section on luster. You see, she told the kids that not all minerals have luster. She equated luster with a sample being shiny. Maybe she was trying to make it simple for the young minds in the classroom, or maybe she really did not know that she was wrong. I’ll never know. I decided to keep my mouth shut because I knew that the kids would learn about minerals again in third grade and having an older child, I knew that that material was correct. Plus, it would just be tacky and rude to stand up and argue with the other scientist.

The fact is that EVERY mineral has a luster. Geologists use so many terms for different types of luster is will make your head spin. Sometimes the luster is obvious and sometimes a little more subtle. Basically, luster is the appearance of a mineral’s surface when light shines on the sample. The absorption, refraction and reflection of the light up on the surface of the mineral will present as different luster.

Some of the most common luster terms you will run across are:

 

Glassy (or vitreous)  – a mineral with a glassy luster shines and reflects light just like real glass.  Examples of minerals with a glassy luster include amethyst, rhodonite, tourmaline, and epidote.  Glassy is the most common luster of all minerals.

 

Pearly – a pearly luster was actually named from the appearance of a real pearl.  Examples of minerals with a pearly luster include talc, lepidolite, and gypsum.

 

Earthy – a mineral with an earthy luster is not shiny and does not reflect light.  The minerals appear dull, like soil. Examples of minerals with an earthy luster include limonite and azurite.

 

Metallic – a mineral with a metallic luster looks like metal (think quarter, pennies, aluminum foil).  Examples of minerals with a metallic luster include galena, pyrite, graphite, and magnetite.

 

Greasy – a mineral with a greasy luster looks as though it were covered with grease.  Examples of minerals with a greasy luster include sulfur, halite, and sodalite.

 

 

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