Boulangerite on Calcite

A slightly curved matrix of solid Calcite whose both top and bottom sides are richly covered by masses of small and shiny Calcite cystals. The remarkable feature of this specimen is that on one side these crystals lie on a layer of Boulangerite, while on the other side these are almost entirely absent. Consequently, where the Boulangerite is present, the side is almost black in appearance, but where the mineral is absent, the Calcite is colourless, but has a pale blue undertone. A very unusual and attractive display specimen.