From ZME Science via reader Ant, we have a priceless and amazing fossil. The description:
Yes, what you are looking at is a natural, though extremely rare phenomenon – quite possibly unique in the world. This fossilised gastropod from the Colombian mine of Gachala has been completely replaced by precious emerald.Formed from hydrothermal fluids in a shear zone interacting with the rock they passed through, the fossil in the host black shale was transformed. If you look carefully you can also see many small crystals of pyrite, a common inclusion in emeralds from this country. Despite its small size (1.3×1.1×0.9Cm), the preservation is near perfect. This fossil is, as far as I know, unique.