Pyrite-bearing rock (likely pyrite in a quartz or sulfide-rich matrix)

The specimen shows numerous small, brassy-yellow, metallic-looking grains in a dark gray matrix, strongly suggestive of pyrite. Pyrite has a Mohs hardness of 6–6.5, metallic luster, pale brass-yellow color, black to greenish-black streak, cubic or pyritohedral crystal structure, and no distinct cleavage. Specific gravity is about 5.0. The photograph is somewhat out of focus, so chalcopyrite or another metallic sulfide cannot be ruled out without testing.

Pyrite commonly forms when iron-bearing fluids react with sulfur in hydrothermal veins, sedimentary rocks, metamorphic rocks, and some igneous systems. It may also form during burial and diagenesis in oxygen-poor marine sediments. Its age depends on the host rock and could range from recent geological formation to billions of years old; the image alone cannot establish a precise age.

Most pyrite specimens are collected for their metallic crystals and geological interest. Historically, pyrite was used as a source of sulfur and occasionally for iron, though it is now economically important mainly as an accessory mineral associated with ore deposits. It is generally not used as a gemstone because it is brittle and can tarnish or oxidize.

Pyrite is commonly called “fool’s gold,” but it is harder and usually more brittle than gold. Unlike gold, pyrite typically produces a dark streak, may show cubic crystal faces, and often fractures rather than bends. Oxidation of pyrite can produce iron oxides and acidic drainage, especially in exposed mine waste.

In the field, look for a distinctly metallic brass-yellow color, high density, brittle fracture, and cubic, pyritohedral, or granular grains. A streak test on unglazed porcelain should be dark greenish-black to brownish-black, not yellow. Pyrite should not be scratched easily by a steel knife, while chalcopyrite is softer (Mohs 3.5–4) and often has a more golden color with iridescent tarnish. Definitive identification would require a sharper view, hardness and streak tests, density, or laboratory analysis. Common occurrences include hydrothermal veins and sulfide deposits in Peru, Spain, Italy, the United States, Canada, China, and Mexico.