Cassiterite - Lustrous Jet-Black Twinned Crystal Cluster, Pingwu

Cassiterite from the Pingwu Beryl Mine in Sichuan is one of the more quietly desirable Chinese occurrences, better known for its aquamarine and scheelite but capable, in the right pockets, of producing sharply formed tin oxide crystals with a metallic-adamantine polish that few other localities match. This miniature is a compact, radiating cluster of interpenetrant crystals, chosen for lustre rather than size, and it rewards the hand more than the shelf. The architecture is characteristic of the species at its best: stout prismatic individuals terminated by steep pyramidal faces, locked together in the elbow and knee twins that give Cassiterite its distinctive geniculate look. The crystals fan outward from a common growth centre, producing a rounded, rosette-like front face with genuine depth, the smaller individuals stepping back and downward around the periphery rather than lying flat. A dominant tabular crystal occupies the centre of the display face and organises everything around it. In ordinary reflected light the body colour reads as dense jet to brownish-black, near opaque, with the faint reddish-brown undertone that betrays the material's true nature where light catches an edge. Thin blades at the margins glow a deep chestnut when backlit. The lustre is the real argument for this piece: a hard, near-submetallic brilliance that flashes almost silver-white on the broadest faces and skates across the smaller terminations as the specimen is rotated, so that the surface appears to fire in sequence rather than all at once. Cassiterite is hard but brittle, and its projecting pyramidal tips and knife-sharp twin edges are precisely the features that chip in the pocket and in handling. On a piece whose entire appeal rests on unbroken mirror faces, the preservation visible here counts for a great deal. A miniature in the truest sense, and a specimen that repays being turned slowly under a single directional light.