igneous
Quartz-filled Basalt Geode
Amygdaloidal Basalt with Quartz (SiO2)
Hardness: 7 (quartz) vs 5-6 (basalt matrix); Color: Dark grey matrix with white/pink crystalline center; Luster: Vitreous in center, dull in matrix; Structure: Crystalline quartz in a microcrystalline volcanic host; Specific Gravity: Approx 2.7-3.0
- Hardness
- 7 (quartz) vs 5-6 (basalt matrix)
- Color
- Dark grey matrix with white/pink crystalline center
- Luster
- Vitreous in center, dull in matrix
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Physical properties
Hardness: 7 (quartz) vs 5-6 (basalt matrix); Color: Dark grey matrix with white/pink crystalline center; Luster: Vitreous in center, dull in matrix; Structure: Crystalline quartz in a microcrystalline volcanic host; Specific Gravity: Approx 2.7-3.0
Formation & geological history
Formed from volcanic lava cooling (Basalt). Gas bubbles (vesicles) were trapped in the rock and later filled with hydrothermal silica-rich fluids that crystallized into quartz over millions of years.
Uses & applications
Primarily for geological collection, educational purposes, and as a decorative stone or paperweight.
Geological facts
These specimens are often called 'amygdules' (meaning almond-shaped). If the cavity is not completely filled, it is called a geode; if it is solid, it is technically an amygdule.
Field identification & locations
Look for rounded, heavy dark volcanic rocks with white 'eyes' or veins. Often found in areas with historic volcanic activity or in glacial till and riverbeds throughout the Pacific Northwest and Great Lakes regions.
