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Have you ever picked up a pretty rock and wondered if it contained precious gems? Have you ever thought that your sharp eyes could make you a millionaire? How can you tell for sure that you found a gold nugget?
Rocks are made of combination of minerals. If you visit Yosemite National Park, you will find lots of rocks with clear, white, and black flecks. This type rock is called granite, and consists of three minerals: quartz, feldspar, and mica. Minerals have a uniform chemical composition and occur naturally in the Earth’s crust. When geologists try to identify a mineral, they investigate several properties. They might test how hard a mineral is (Talc can be scratched with a fingernail, while a diamond can scratch glass). Geologists also look at the color of the mineral, but they know that color can be misleading! The mineral quartz, for example, is commonly seen in clear, pink, and brown forms. Sometimes, the outer surface of a rock can react with air, giving it a different color. The streak of a mineral is the color of the powdered form of the mineral, and is often a more useful clue than the apparent color of the mineral alone.
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