Is marble, metal or concrete better for a garden cremation urn?

My family and I are planning to keep my late uncle’s ashes in a garden urn as he always loved to be outside. As the cremation urn will be more exposed to the elements, we obviously want to choose a material that’s durable and weather-proof. My question is then is marble, a metal like cast iron or concrete better for my uncle’s urn? Thanks in advance.