Calcite, calcium carbonate, CaCO3 I discovered this unusual geode in an excavation for industrial buildings in Bridgenorth, Shropshire. The excavation was in sandstone which is predominant in Bridgenorth. The outer shell of the geode is composed of rounded and angular sandstone grains of sizes from .5 mm to 3mm set in a calcite cement. The main core is massive calcite, and the crystals of calcite in the centre are mainly indistinct crystals, but with a few obsevable scalenohedral crystals, nearly all with a brown coating.