igneous
Amygdaloidal Basalt with Quartz Infill
Basalt (SiO2 for the quartz inclusion)
Hardness: 6-7 (basalt matrix) 7 (quartz); Color: Dark gray to black matrix with white crystalline pocket; Luster: Dull/earthy matrix with vitreous inclusions; Crystal structure: Microcrystalline matrix with trigonal quartz crystals; Cleavage: None.
- Hardness
- 6-7 (basalt matrix) 7 (quartz)
- Color
- Dark gray to black matrix with white crystalline pocket
- Luster
- Dull/earthy matrix with vitreous inclusions
Identified More igneous →
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Physical properties
Hardness: 6-7 (basalt matrix) 7 (quartz); Color: Dark gray to black matrix with white crystalline pocket; Luster: Dull/earthy matrix with vitreous inclusions; Crystal structure: Microcrystalline matrix with trigonal quartz crystals; Cleavage: None.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The white 'amygdule' or cavity formed when gas bubbles were trapped in cooling lava and later filled by silica-rich hydrothermal fluids over thousands to millions of years.
Uses & applications
Basalt is used extensively in construction (crushed stone, asphalt). Specimens with mineral-filled cavities are prized by amateur collectors and used for educational purposes.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. When gas bubbles in lava are filled with secondary minerals, they are called amygdules.
Field identification & locations
Identify by its heavy weight, dark fine-grained appearance, and the presence of 'pockets' of white or clear crystals. Can be found in volcanic regions such as the Pacific Northwest or the Lake Superior region.
