Smithsonite mineral sample. This mineral is a form of zinc carbonate, and is also called zinc-spar. It is most often found in its botryoidal form, but also forms crystals. It is usually green, but other colours are possible, including pink and off-white. Here, the crystals are better developed than usual for this mineral. This sample, from Elkhorn in Montana, USA, is 6.5 centimetres across.