Pyrite in Basalt (Fool's Gold)
Pyrite (Iron Disulfide) · Mineral Specimen / Industrial Mineral
Crystal System
Cubic (Isometric); typical habits include cubes, pyritohedrons, and octahedral crystals, often appearing as granular or massive aggregates within rock matrices.
Mineral Group
Sulfides
Chemical Formula
FeS2
Formation Process
Hydrothermal or Magmatic; forms in igneous environments through the cooling of sulfur-rich magma or from hydrothermal fluids filling cavities in volcanic host rock like basalt.
About This Crystal
The specimen features brassy, metallic yellow pyrite crystals embedded in a dark, vesicular gray-black basaltic matrix. The pyrite displays a metallic luster and opaque transparency, appearing as small granular clusters or micro-crystals.
Physical Characteristics
Color: Brass-yellow; Streak: Greenish-black to brownish-black; Cleavage: Indistinct; Fracture: Conchoidal to uneven; Specific Gravity: 4.9–5.2; Tenacity: Brittle. Generally non-fluorescent.
Optical Properties
Opaque; metallic luster. Shows very high reflectivity. Since it is opaque, it does not have a refractive index in the standard sense (refractive index is N/A for metals/sulfides).
Hardness & Durability
Origin Region
Globally distributed; notable sources include Spain (Rio Tinto), Italy (Elba), Peru, and the USA. This specimen appears as an inclusion in a volcanic rock (basalt).
Hardness & Durability
Mohs Hardness: 6 to 6.5. It is relatively hard and can scratch glass, but it is brittle and can crack or crumble under pressure or impact.
Care & Maintenance
Keep dry; pyrite can oxidize (pyrite disease) in high humidity, turning into sulfuric acid and iron sulfates. Clean with a dry brush or soft cloth. Avoid ultrasonic cleaners and acidic chemicals.
Rarity & Value
Common; widely available. Value for this type of specimen is generally low, determined by the size of the pyrite clusters and the aesthetic contrast with the host rock ($5–$30 for average hand specimens).
Special Characteristics
Metallic luster with a distinctive brassy color that mimics gold. Pyrite is paramagnetic and can produce sparks when struck against steel (giving it the name 'fire stone').
Lore & History
Historically used by ancient Romans to produce sparks for fire. In lore, it is used as a stone of protection and wealth. Known as 'Fool\'s Gold' because prospectors often confused it with real gold.