Crystals
Quartz
Silicon dioxide SiO2 Trigonal crystal system at ordinary conditions Hexagonal prisms with rhombohedral terminations Mohs hardness 7 Specific gravity about 2.65 Uniaxial positive and optically active Piezoelectric and widely used...
Quartz
Silicon dioxide SiO2 Trigonal crystal system at ordinary conditions Hexagonal prisms with rhombohedral terminations Mohs hardness 7 Specific gravity about 2.65 Uniaxial positive and optically active Piezoelectric and widely used...
Pyrite
Iron disulfide FeS2 Cubic crystal system Cubes, pyritohedra, octahedra, and combinations Mohs hardness 6–6.5 Specific gravity about 5.0 Metallic brass-yellow luster Greenish-black to brownish-black streak Hydrothermal, sedimentary, igneous, and metamorphic...
Pyrite
Iron disulfide FeS2 Cubic crystal system Cubes, pyritohedra, octahedra, and combinations Mohs hardness 6–6.5 Specific gravity about 5.0 Metallic brass-yellow luster Greenish-black to brownish-black streak Hydrothermal, sedimentary, igneous, and metamorphic...
Prehnite
Calcium–aluminum silicate hydroxide Ca2Al2Si3O10(OH)2 Orthorhombic crystal system Botryoidal, fanlike, reniform, and stalactitic habits Mohs hardness 6–6.5 Specific gravity 2.80–2.95 Biaxial positive optical character Mafic-rock cavities and low-grade metamorphism Often associated...
Prehnite
Calcium–aluminum silicate hydroxide Ca2Al2Si3O10(OH)2 Orthorhombic crystal system Botryoidal, fanlike, reniform, and stalactitic habits Mohs hardness 6–6.5 Specific gravity 2.80–2.95 Biaxial positive optical character Mafic-rock cavities and low-grade metamorphism Often associated...
Porphyry
Igneous rock texture Phenocrysts in fine groundmass Felsic to mafic compositions Volcanic and shallow intrusive Quartz, feldspar, amphibole, pyroxene, or olivine Two-stage model with complex crystal histories Imperial purple porphyry...
Porphyry
Igneous rock texture Phenocrysts in fine groundmass Felsic to mafic compositions Volcanic and shallow intrusive Quartz, feldspar, amphibole, pyroxene, or olivine Two-stage model with complex crystal histories Imperial purple porphyry...
Peridot
Gem variety of olivine (Mg,Fe)2SiO4 Orthorhombic nesosilicate Mohs 6.5–7 Idiochromatic yellow-green Strong double refraction Upper mantle and pallasites August birthstone Peridot: Green Light from Earth’s Mantle Peridot is the transparent...
Peridot
Gem variety of olivine (Mg,Fe)2SiO4 Orthorhombic nesosilicate Mohs 6.5–7 Idiochromatic yellow-green Strong double refraction Upper mantle and pallasites August birthstone Peridot: Green Light from Earth’s Mantle Peridot is the transparent...
Orthocera
Fossil straight-shelled cephalopod Commercial names: Orthoceras and “Orthocera” True Orthoceras: Middle Ordovician Baltoscandia Common decorative material: Silurian–Devonian Morocco Chambered orthocone with siphuncle Calcite fossil in dark limestone Matrix hardness approximately...
Orthocera
Fossil straight-shelled cephalopod Commercial names: Orthoceras and “Orthocera” True Orthoceras: Middle Ordovician Baltoscandia Common decorative material: Silurian–Devonian Morocco Chambered orthocone with siphuncle Calcite fossil in dark limestone Matrix hardness approximately...