Bournonite with Quartz - Striated Cogwheel Twin with Terminated Quartz, Yaogangxian Mine

Bournonite from the Yaogangxian Mine in Hunan is the standard against which the species is measured worldwide, and this compact thumbnail carries the two features collectors look for first: the blocky, deeply striated cogwheel architecture, and a contrasting water-clear Quartz crystal perched on the display face. The piece is essentially a single dense block of interpenetrant Bournonite, built from tabular individuals stacked and rotated against one another in the repeated twinning that gives the species its "cogwheel" or Radelerz name. Seen from the front, the crystals present a palisade of parallel, sharply grooved prism faces; seen from above, the same growth resolves into the stepped, notched terraces and re-entrant angles that betray the twin law directly. Little square and rectangular etch-cavities punctuate the surface, adding depth and reinforcing the impression of a structure assembled rather than merely grown. Despite its small footprint the specimen is genuinely three-dimensional, with crystals projecting on more than one face. Colour is a cool, dark steel-grey verging on lead-grey, essentially opaque, warming to a faint bronze cast where oxidation has touched the highest edges. The metallic lustre is the strongest asset here. Rotated under a single directional light, the striated faces flare in sequence, each groove catching the beam in turn so the crystal appears to ripple; the broader cleavage-bright terraces on the top flash a bright mirror silver. That behaviour in the hand is precisely what separates good Bournonite from the dull, greyed-over material that makes up most production. The Quartz is the counterpoint. A doubly visible, nicely terminated colourless prism lies across the front face, glassy and translucent where the Bournonite is opaque and reflective, with light passing cleanly through the termination and glancing off the rhombohedral faces. A scatter of smaller Quartz points and a fine olive-green micro-crystalline dusting occupy the recesses between crystals, softening the transitions and confirming an undisturbed, natural pocket surface rather than a cleaned-down fragment. Condition deserves comment because Bournonite is soft, brittle and easily bruised: the thin projecting twin lamellae and the knife-edge corners of the cogwheels are the first things to chip, and lustre is readily dulled by handling. Here the principal faces read bright and the twin edges are legible, while the reverse shows the natural contact and matrix attachment where the piece grew. At 2.5 x 2.2 x 2.1 cm this is a large thumbnail, blocky and self-contained, with more visual weight than its size suggests. Under a single lamp it turns from dark metal to a wheel of light.