from a private collection in Liguria

A typical association found in copper deposits in the Democratic Republic of the Congo (Katanga, Kolwezi District) and Zambia, where a crust of silicified chalcedony overlies a base of botryoidal malachite, upon which rhombohedral or scalenohedral crystals of transparent calcite were subsequently deposited. The sequence reflects several phases of supergene alteration in an oxidizing environment: first, the formation of malachite through the oxidation of primary copper sulfides; then, superficial silicification by silica-rich solutions (chalcedony); and finally, the late precipitation of calcite from low-temperature carbonate fluids. Variable hardness: 3.5–4 on the Mohs scale (malachite), 6.5–7 on the Mohs scale (chalcedony), 3 on the Mohs scale (calcite).

A specimen of great interest to collectors and museums due to its mineralogical stratigraphy, visible to the naked eye, which tells the geochemical history of the deposit; also used as a valuable decorative piece; sometimes cut and polished cross-sectionally to highlight the color bands.