The building blocks of rocks that are around us are called minerals. Granite for example is formed mainly of minerals quartz and feldspar. Minerals include such common, everyday substances as salt, the graphite in pencil lead and the talc used to make talcum powder. But they also include rarities such as silver, gold and diamond. Despite their variety, all these substances have lot of common traits. Each is a solid with several uniform chemical composition, they all occur naturally on earth; each has a unique crystal structure. Today there are more than 2000 minerals though only 20 of them make up more than 95% of earth’s solid crust.
A few of the world’s treasured ornamental stones - pearls, coral, amber and jet are plant and animal products. But the vast majority of the precious and semi precious stones called gems are minerals.
A mineral’s luster is the way it reflects light. It may be glassy, metallic, pearly or waxy. Hardness is determined by a specimen’s ability to scratch certain other minerals whose hardnesses are rated on a scale of 1 to 10. Talc being the softest among minerals has a hardness of 1 and 10 is reserved for diamonds, the hardest material in nature.
When minerals are scratched against a rough white surface many of them leave a distinctive streak of color. The commonest iron ore hematite varies from brownish red to black. But it always leaves a red streak.
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