Streak Testing

Streaks: It was discovered a long time ago by mineralogists that when a mineral was pulverized to a powder the color it had often times changed noticeably. For example, black hematite will turn into a reddish brown color when it is pulverized. In order to standardize this type of identification, minerals are drawn across a pieced of unglazed tile that is known as a streak plate.

What takes place in this test is best described as drawing a charcoal pencil across a piece of rough drawing paper and it changes a lot of the charcoal into a line of black powder. However in this case a piece of mineral is used to draw across the unglazed tile and the powder line that comes about is what is known as the streak. The color of the streak is then checked against the tables for several minerals and more elimination of possibilities is made.

If a streak does not occur during the test the material is most likely harder than the seven because this is usually the hardness of the streak plate. In actuality almost any piece of tile can be used for this specific test, it is suggested to get a standard tile from a rock shop or a prospecting shop due to the fact that they are the right color. Due to the fact that the powder is usually seen on the streak plate, any color in the back of the tile can contribute to the ending result of the color of the streak.

Luster: When minerals are looked at in reflected light they look as if they revealed a certain luster. It is quite easy to judge some types of luster, for example those that are metallic and dull. However there are other types of luster that need to be tested and it is important to learn how to differentiate them.

One of the best ways you can learn to identify luster is by getting a hold of several know of mineral specimens that have all different types of luster and keep hold of them so they can serve you as samples.

Remember also that the lighting is also a big factor and when testing them you will need to work in the same type of lighting. Bright lights and artificial lights can change the luster a lot. If the specimen is checked on a cloudy day, or on a day when the sun is shining, or in the shade etc, it the luster on it will look different in all the cases.

In general, luster is separated into two different classifications, which are metallic and nonmetallic. The nonmetallic type has a great number of subdivisions and the testing is actually focused on this one. The following are a few of the classifications and each of them describes the example of their luster:

Metallic: This is shiny like other metals in their natural form such as gold and silver.

Submetallic: This is an arrangement between nonmetallic and metallic.

Adamantine (Nonmetallic): has a bright glossy luster to it like in the case of diamonds.

Dull (Nonmetallic): Is very dull in its appearance and can be seen in clay or chalk.

Greasy (Nonmetallic): This has a greasy or oily surface such as in nepheline.

Pearly (Nonmetallic): This has a silky or rayon feel to it such as can be found in abestos.

Resinous (Nonmetallic): Is resinous such as can be found in sphalerite.

Silk (Nonmetallic): Similar to rayon or silk such as can be found in abestos.

Viterous (Nonmetallic): This is similar to glass such as can be found in topaz and quartz.