Overview

Bournonite (PbCuSbS3) is a classic lead–copper–antimony sulfosalt celebrated for its distinctive cogwheel twins—repeatedly twinned crystals that resemble tiny gear wheels. Typically steel-gray to black with a metallic luster, crystals can be blocky to tabular and often show fine striations. Because it forms in hydrothermal veins with other sulfides and sulfosalts, it is frequently found on contrasting matrix such as snowy quartz or honey-brown siderite, and with companions like tetrahedrite, galena, sphalerite, stibnite, fluorite, or barite. Fine bournonite has a “machined” look—sharp, lustrous faces and crisp twin notches. Aesthetic matrix pieces with complete, undamaged cogwheels are the benchmark and can be breathtaking cabinet centerpieces.

Bournonite is relatively soft and brittle (Mohs ~2.5–3), heavy (SG ~5.8–5.9), and can tarnish, developing iridescent films. The species combines historical cachet—famed from European classic districts—with modern finds that brought superb association pieces to market. Collectors prize it for the iconic twinning, metallic brilliance, and striking contrast when perched on white quartz or alongside translucent fluorite.

Popularity

Among sulfide and sulfosalt collectors, bournonite holds a special, enduring appeal. The 19th-century “cogwheels” from Cornwall and central Europe became Victorian collection staples and still define the species’ image. Later discoveries in Bolivia and Peru produced larger crystals and dramatic clusters, while China’s Yaogangxian Mine yielded modern combinations with fluorite and quartz that captivated a new generation of collectors.

Entry-level specimens include small single crystals and thumbnails with modest luster; prices rise quickly for sharp twinned “gears,” complete rosettes, and well-composed matrix pieces. Herodsfoot Mine wheels or large Peruvian/Bolivian twins are competition-grade and command strong premiums. Because many crystals are fragile and often damaged in the pocket, pristine undamaged pieces are scarce, placing the very best in a rarified tier. The species remains a cornerstone for anyone building a classic metallic suite or a sulfosalt specialization.

Top Collecting Localities

Below are renowned sources of bournonite with strong provenance and distinctive styles. Each locality’s identity rests on habit, associations, and historic significance, and all have produced specimens that set standards for the species.

Herodsfoot Mine, Cornwall, UK

Perhaps the most iconic bournonites of all time are the Herodsfoot “cogwheels” from Cornwall. The mine’s 19th-century pockets produced perfect gear-shaped twins—often 1–3 cm across, sometimes larger—set on drusy quartz or with sulfide matrix. These wheels show razor-sharp notches, classic steely luster, and an aesthetic balance rarely matched. Complete, undamaged wheels on attractive matrix are trophy pieces in major collections. Even today, a fine Herodsfoot thumbnail can outshine much larger examples thanks to peerless form and history.

Yaogangxian Mine, China

China’s Yaogangxian Mine brought modern, highly aesthetic bournonite to market, frequently in showy combinations with fluorite cubes, quartz, and sulfides. Crystals are usually smaller than Bolivian wheels but often extremely sharp, lustrous, and well-arranged on contrasting matrix. The fluorite association—purple to green, sometimes zoned—creates color contrast that adds significant display impact. Fine, unrepaired matrix pieces are especially sought after.

Collector’s Guide

Evaluating Specimen Quality

• Color and Luster: Bournonite should appear steel-gray to black with a fresh, metallic luster. Bright, reflective faces elevate a piece; dull, matte surfaces or heavy oxidation diminish appeal. A natural soft patina is acceptable, but avoid specimens with extensive alteration crusts unless historic or otherwise exceptional.

• Twinning and Crystal Form: The “cogwheel” twin habit is the species’ signature. Desirability rises with sharp gear notches, complete symmetry, and intact wheels. Non-twinned crystals can still be outstanding if they are sharply formed, well-terminated, and lustrous. Look for crisp edges, clean striations, and proportionate thickness—stout, “machined” crystals read best in a case.

• Completeness and Damage: Because bournonite is fragile, many crystals are nicked or bruised, especially on the teeth of cogwheels. Premium pieces have little to no damage on the main crystals and present well from the prime display angle. Chips along edges show as bright, fresh-looking breaks and can distract even at thumbnail scale.

• Size versus Aesthetics: Large wheels are rare and valuable, but aesthetics rules—an exquisitely sharp 2–3 cm wheel can outshine a bigger but dull or dinged piece. Clusters should be balanced and dynamic rather than jumbled; a single dominant wheel with supporting crystals often displays better than a crowded spray.

• Matrix and Associations: Strong contrast greatly enhances value. Classic white quartz, creamy barite, or pastel fluorite makes the metallic bournonite pop. Clean, undamaged matrix with attractive accessory minerals (tetrahedrite, sphalerite, galena, siderite) adds context and visual depth. True matrix specimens carry a premium over loose crystals.

• Provenance: Historical districts (Herodsfoot, Freiberg, Příbram) and modern benchmark finds (Chocaya, Julcani, Yaogangxian) command higher interest. Labels and documented provenance can significantly increase desirability and value, especially for classic European material.

Detecting Repairs or Treatments

• Repairs and Reattachments: Due to brittleness, crystals or wheels are sometimes reattached to matrix or repaired across a clean break. Inspect for perfectly straight junctions, subtle misalignment of striations, or glossy glue lines. A UV check may reveal fluorescent adhesive. Quality repairs can be acceptable if disclosed; multiple repairs lower value.

• Polishing and Surface Alterations: Polished or buffed faces lose natural growth texture and can look “too smooth” or glassy compared to adjacent faces. Avoid overly “plastic” sheen in a localized area—it may indicate polishing or resin fill.

• Chemical Cleaning: Aggressive acid cleaning used to strip carbonates can etch bournonite, dulling luster or leaving a granular surface. Prefer specimens with natural, unetched metallic faces; if matrix suggests heavy acid use, scrutinize the bournonite surfaces carefully.

• Fakes: Wholesale fakes are uncommon. Watch for assembled “marriages” of unrelated crystals on mismatched matrix. Natural bournonite shows growth lines, striations, and realistic contact areas—not perfectly clean, featureless interfaces.

Care and Storage

Bournonite is soft, brittle, and prone to tarnish. Thoughtful care preserves luster and prevents damage.

• Handling: Support from the matrix; avoid pinching crystal edges or cogwheel teeth. Use two hands for larger pieces. Do not test “firmness” by pressing wheels—they can snap.

• Environment: Store in a dry, stable environment. Low humidity (with silica gel desiccant) helps reduce tarnish and sulfide oxidation. Avoid proximity to reactive sulfur sources and off-gassing woods or foams. Normal light is fine; avoid heat sources and large temperature swings that can stress crystals or adhesives.

• Cleaning: Dust gently with a soft brush or air bulb. If necessary, use room-temperature distilled water with a drop of mild soap and a very soft brush, then rinse with distilled water and dry thoroughly. Never use ultrasonic cleaners, steamers, or harsh acids—these can crack or etch bournonite and damage associated matrix minerals. Do not long-soak pieces with delicate matrices.

• Display: LED case lighting is ideal. Keep pieces secured with museum putty or a custom stand; vibrations can walk specimens off shelves. For pieces on pyrite/marcasite matrix, prioritize very low humidity storage to mitigate “pyrite disease.”

• Long-Term Preservation: If slight tarnish appears and you prefer a bright look, consider professional conservation rather than aggressive DIY methods. Accept that many classic pieces develop a gentle patina over time; it is often part of their charm and history.