igneous

Granitic Gravel Piece

Granite (Felsic Plutonic Rock / SiO2 with Al2O3, K2O, Na2O)

Hardness: 6-7 (Mohs scale); Color: Mottled cream, white, and dark grey; Luster: Dull to vitreous; Crystal structure: Phaneritic (visible interlocking crystals); Specific gravity: 2.63 - 2.75.

Hardness: 6-7 (Mohs scale)
Color: Mottled cream, white, and dark grey
Luster: Dull to vitreous

Identified March 29, 2026More igneous →

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Physical properties

Hardness: 6-7 (Mohs scale); Color: Mottled cream, white, and dark grey; Luster: Dull to vitreous; Crystal structure: Phaneritic (visible interlocking crystals); Specific gravity: 2.63 - 2.75.

Formation & geological history

Formed from the slow cooling of silica-rich magma deep underground (plutonic). This process can occur over millions of years, often dating back to the Precambrian or Paleozoic eras, later exposed by erosion.

Uses & applications

Commonly used in construction aggregate, road base, and landscaping gravel. As a larger slab, granite is prized for countertops and ornamental stone.

Geological facts

Granite makes up a large portion of the Earth's continental crust. Because it is highly resistant to acid rain and weathering, it has been used since ancient times for monuments, including the Great Pyramid of Giza's burial chamber.

Field identification & locations

Identify by its 'salt and pepper' appearance of interlocking light (quartz/feldspar) and dark (biotite/hornblende) minerals. Found globally in mountain ranges and eroded shield areas. It will not react with vinegar (differentiating it from limestone).

Other igneous specimens

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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)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral