Copper rich calcite helictite, Bisbee, Arizona, USA
Helictites are similar to stalagmites; they are exuded and grow from the walls of caves, but form angular and helical structures. This helictite is copper rich, hence the green tint. It is reactive to UV and must be kept out of direct sunlight, or it will discolour.
The photograph is of the item which is offered for sale.