This is a crystal of olivine within the igneous rock basalt.
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Image from: Andrew Alden, About.com Geology Guide
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Olivine
Olivine looks like little green crystals. It is typically found in some igneous and metamorphic rocks. Often the crystals are so small that you need to use your hand lens or magnifying glass to see them clearly.
- Shape: Orthorhombic (usually a many-sided prism that has an overall sphere shape)
- Luster: Greasy
- Color: Green (but sometimes yellow or brown)
- Streak: White
- Hardness: 6.5-7 on Mohs Hardness Scale
- Cleavage: Yes, but the crystals do not break easily along it
- Fracture: Conchoidal, brittle
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