igneous

Decomposed Granite

Grano-diorite/Granite-residuum (SiO2 primary)

Hardness: 6-7 (individual quartz/feldspar grains); Color: Tan, buff, gray, pinkish; Luster: Pearly to dull; Structure: Granular/fragmented; Cleavage: None (in bulk state); Specific Gravity: 2.6-2.7

Hardness: 6-7 (individual quartz/feldspar grains)
Color: Tan, buff, gray, pinkish
Luster: Pearly to dull

Identified February 17, 2026More igneous →

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Physical properties

Hardness: 6-7 (individual quartz/feldspar grains); Color: Tan, buff, gray, pinkish; Luster: Pearly to dull; Structure: Granular/fragmented; Cleavage: None (in bulk state); Specific Gravity: 2.6-2.7

Formation & geological history

Formed through the natural chemical and physical weathering of granitic rock over thousands of years; typically found in arid or semi-arid environments where rocks fracture into gravel and sand.

Uses & applications

Extensively used in landscaping, driveway surfacing, walking paths, and as a stable base for pavers. It is valued for its permeability and natural aesthetic.

Geological facts

Geologists often refer to this material as 'grus'. It is a signature feature of the Mojave and Sonoran Deserts, where vertical granite outcroppings crumble into large aprons known as bajadas.

Field identification & locations

Identify in the field by its sandy, gritty texture and the presence of small, angular chunks of quartz and potassium feldspar. Commonly found throughout California, Arizona, and the American Southwest.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

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