Diorite (Phaneritic Igneous Rock)
Diorite (Intermediate Plutonic Rock composed of Plagioclase and Hornblende/Biotite) · Decorative Stone / Mineral Specimen / Plutonic Igneous Rock
Crystal System
Polycrystalline/Aggregate. Individual minerals within are Triclinic (Feldspar) and Monoclinic (Amphibole). The habit is phaneritic (coarse-grained) and massive.
Mineral Group
Silicates (Subgroups: Tectosilicates and Inosilicates)
Chemical Formula
Variable; primarily (Na,Ca)(Si,Al)4O8 (Plagioclase) + (Ca,Mg,Fe,Al) silicate (Amphibole group)
Formation Process
Igneous: Formed by the slow cooling of magma deep beneath the Earth's surface (intrusive) at moderate temperatures and high pressures, typically in subduction zones.
About This Crystal
A salt-and-pepper appearance characterized by equigranular crystals of white plagioclase feldspar and dark green to black hornblende or biotite. It is opaque with a dull to vitreous luster on fresh surfaces and a speckled, mottled texture.
Physical Characteristics
Color: Mottled white and black/dark grey. Streak: Usually white (from feldspar). Cleavage: Good in two directions for internal mineral grains. Specific gravity: 2.8 - 3.0. Non-fluorescent. Tenacity: Brittle but very tough.
Optical Properties
Opaque in hand specimen. Under microscope (thin section), minerals show high birefringence (amphiboles) and polysynthetic twinning (plagioclase). Non-phenomenal.
Hardness & Durability
Origin Region
Globally distributed; common in volcanic arcs and cordilleran mountain building zones such as the Andes, Italian Alps, and British Columbia.
Hardness & Durability
Mohs hardness: 6 to 7. Highly durable and resistant to weathering; historically used as a building material and for sculpture due to its extreme toughness.
Care & Maintenance
Clean with warm soapy water and a soft brush. Resistant to most household chemicals but should be stored away from harder gemstones like Corundum to prevent scratching.
Rarity & Value
Common. Value is generally low for raw specimens; higher value is found in polished slabs for countertops or carved decorative objects ('Black Granite' trade name).
Special Characteristics
Phaneritic texture (visible crystals) creates a classic 'salt and pepper' look. High resistance to compression and abrasion.
Lore & History
Historically significant as the medium for the Code of Hammurabi (ancient Babylonian law). In metaphysical lore, it is believed to be a grounding stone that helps in 'bridging' different perspectives and provides stability during chaotic times.