High School Earth Science: Earth’s Minerals - Mineral Identification — Flashcards

Diamonds are popular gemstones because of the way they reflect light. Their luster makes them appear very sparkly.    
Turquoise is prized for its color. It is a striking greenish blue. The color of minerals and how they reflect light are two physical properties that may help to identify minerals.    
Although the color of a mineral is easy to observe, color alone is rarely very useful for identifying minerals. One reason is that different samples of the same mineral may vary in color. For example, different samples of quartz may be colorless (clear) or purple. Purple quartz contains tiny amounts of iron that give it its color. Many minerals are colored by chemical impurities in this way. Another reason that color alone may not be very useful is that different minerals may be the same color. For example, both gold and pyrite (“fool’s gold”) are yellowish gold in color, but the two are different minerals.    
A more useful property for identifying many minerals is streak. Streak is the color of a mineral’s powder. To check a mineral’s streak, scrape a piece of the mineral across an unglazed porcelain plate. Streak is a more reliable property than color for mineral identification because a given mineral’s streak does not vary. Even minerals that are the same color differ in the color of their streak. For example, gold has a golden yellow streak, whereas pyrite has a blackish streak. Streak cannot be used to identify all minerals because not all minerals leave a streak of powder when scraped across unglazed porcelain. Quartz is an example of a mineral that does not have streak.    
Luster describes how light reflects off a mineral’s surface. Mineralogists have special terms to describe luster. First, they divide all minerals into those with metallic luster and those with non-metallic luster. Minerals with metallic luster are opaque and shiny. Pyrite is an example. All other minerals have non-metallic luster. There are six different types of non-metallic luster, including adamantine luster, which appears sparkly. Diamond has adamantine luster.    
Other types of non-metallic luster are earthy luster, which appears dull and clay-like; pearly luster, which appears pearl-like; resinous luster, which appears resin-like, as in tree resin; silky luster, which appears soft looking with long fibers; and vitreous luster, which appears glassy.