igneous

Demitasse Granite (Diorite-Granite transition)

Intermediate Plutonic Rock (Granodiorite/Diorite mix)

Hardness: 6-7 Mohs; Color: Mottled black, olive-green, and off-white; Luster: Vitreous to dull when wet; Structure: Phaneritic (visible crystals); Cleavage: Perfect in feldspar laths; Specific Gravity: 2.7-3.0

Hardness: 6-7 Mohs
Color: Mottled black, olive-green, and off-white
Luster: Vitreous to dull when wet

Identified April 5, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow cooling of magma beneath the Earth's surface (intrusive). The dark minerals are likely amphibole or pyroxene, while the lighter green/tan is plagioclase feldspar. Common in continental crust and mountain ranges.

Uses & applications

Used primarily as crushed stone for construction, decorative gravel, and occasionally as architectural stone in larger slabs.

Geological facts

Rocks with this 'salt and pepper' appearance often represent a 'frozen' state of magma mixing. The green tint suggests mild hydrothermal alteration of the mafic minerals into chlorite or epidote group minerals.

Field identification & locations

Identify in the field by the interlocking crystalline texture and lack of layering (foliation). Look for it in riverbeds near mountainous or volcanic roots. It will not scratch with a pocket knife.

Other igneous specimens

See all

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