Igneous

Quartz in Basalt (Xenocryst)

Silicon Dioxide (SiO2) in Mafic Igneous Volcanic Rock

Hardness: 7 (quartz) vs 5-6 (basalt matrix); Color: Milky white crystal in dark charcoal/black matrix; Luster: Vitreous quartz in dull/earthy matrix; Crystal structure: Hexagonal quartz.

Hardness: 7 (quartz) vs 5-6 (basalt matrix)
Color: Milky white crystal in dark charcoal/black matrix
Luster: Vitreous quartz in dull/earthy matrix

Identified March 27, 2026More igneous →

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Physical properties

Hardness: 7 (quartz) vs 5-6 (basalt matrix); Color: Milky white crystal in dark charcoal/black matrix; Luster: Vitreous quartz in dull/earthy matrix; Crystal structure: Hexagonal quartz.

Formation & geological history

Formed when quartz crystals (either from previous cooling cycles or crustal contamination) are engulfed by rising basaltic lava. Most common in Cenozoic era volcanic flows. The surrounding matrix cooled quickly at the surface.

Uses & applications

Primarily used for geological study of volcanic plumbing systems; basalt is used in construction aggregate and road base.

Geological facts

This specimen is technically a 'xenocryst'—a crystal that is foreign to the igneous rock in which it is embedded. Quartz is actually rare in basalt because the chemistry of the lava usually dissolves it.

Field identification & locations

Identify by the sharp contrast between the light, translucent mineral and the dark, fine-grained host rock. Commonly found in volcanic fields such as those in the Pacific Northwest USA, Iceland, or the Deccan Traps.

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