Pyrargyrite Identification Guide

What Pyrargyrite Looks Like

Pyrargyrite (Ag3SbS3, "dark ruby silver") is a silver antimony sulfosalt and an important silver ore. Fresh crystals are deep cochineal-red to grayish-black, often appearing nearly black in bulk but flashing deep red where light passes through thin edges or splinters. Luster is adamantine to submetallic, and crystals are translucent to nearly opaque. It commonly forms prismatic, scalenohedral, or rhombohedral crystals in the trigonal system, but also occurs massive, compact, or as crusts. Surfaces frequently dull and darken on long light exposure.

Step-by-Step Field ID Checklist

1. Note the color: brownish-black to gray with a deep red internal glow. Hold a thin sliver to strong light to catch the ruby-red transmission.
2. Check the luster: adamantine to metallic-adamantine, not earthy.
3. Scratch a streak: it should be purplish-red to cochineal-red — the single most diagnostic test.
4. Test hardness: a soft 2.5, easily scratched by a knife.
5. Heft it: density is high (~5.8), noticeably heavy for its size.
6. Look at the setting: associated with native silver, acanthite, galena, and other silver sulfosalts in hydrothermal veins.

Key Diagnostic Tests

• Streak: purplish-red — distinguishes it from black opaque sulfides whose streaks are black or gray.
• Hardness: 2.5 (Mohs); a steel knife scratches it readily.
• Specific gravity: ~5.85, distinctly heavy.
• Cleavage/fracture: distinct rhombohedral cleavage; conchoidal to uneven fracture; brittle.
• Light sensitivity: darkens over time when exposed to light — store in the dark.
• No magnetism; no acid reaction of note in the field.

Common Look-Alikes and How to Tell Them Apart

• Proustite (light ruby silver, Ag3AsS3): the closest twin. Proustite is brighter scarlet-red with a vermilion (lighter red) streak, while pyrargyrite is darker with a purplish-red streak. Chemistry (As vs Sb) is the difference; color and streak shade are the field clues.
• Cinnabar: also red with a red streak, but cinnabar is even softer (2–2.5), brighter scarlet, and lacks the metallic-adamantine luster and silver associations.
• Cuprite: red and heavy but harder (3.5–4) with a brownish-red streak and copper associations.
• Hematite: harder (5–6) with a cherry-red to red-brown streak and no silver ore association.
• Acanthite/galena: black with gray-black streaks, no red transmission.

Where Pyrargyrite Is Found

It occurs in low-temperature hydrothermal silver veins, typically as a late-stage mineral. Classic localities include the silver districts of Mexico (Guanajuato, Zacatecas, Fresnillo), the Harz Mountains and Freiberg in Germany, Bolivia, Peru, the Czech Republic, and silver camps of the western United States. Look for it on vein quartz or carbonate gangue alongside native silver and acanthite.