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

Identified March 18, 2026More igneous →

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Physical properties

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.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

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Magnetite

Magnetite (Fe3O4)

mineral