igneous
Granitic Pegmatite (River Rock specimen)
Granite / Pegmatite (composed of SiO2, KAlSi3O8, and NaAlSi3O8)
Hardness: 6-7 (Mohs scale); Color: White, cream, and grey with black inclusions; Luster: Vitreous to pearly; Crystal structure: Macro-crystalline phaneritic; Cleavage: Poor (quartz) to perfect in two directions (feldspar); Specific gravity: 2.6-2.7.
- Hardness
- 6-7 (Mohs scale)
- Color
- White, cream, and grey with black inclusions
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: White, cream, and grey with black inclusions; Luster: Vitreous to pearly; Crystal structure: Macro-crystalline phaneritic; Cleavage: Poor (quartz) to perfect in two directions (feldspar); Specific gravity: 2.6-2.7.
Formation & geological history
Formed from the slow cooling of silica-rich magma deep within the Earth's crust, typically during the final stages of crystallization. This specific specimen has been rounded and smoothed by water transport (fluvial erosion), turning a jagged rock fragment into a 'river cobble'.
Uses & applications
Crushed for construction aggregate, decorative landscaping, or as a source of high-purity quartz and feldspar for ceramics and industrial glass.
Geological facts
Pegmatites are famous for containing exceptionally large crystals, sometimes reaching several meters in size. They are often the primary source of gemstones like tourmaline, beryl, and topaz, though this specimen primarily shows quartz and feldspar.
Field identification & locations
Identify in the field by the large, distinct interlocking crystals of translucent quartz (greyish) and opaque feldspar (white/tan). It will not be scratched by a steel knife. Common in riverbeds draining from mountainous or shield regions.
