Absolutely! Traditionally stone monuments in Europe and North America were made from marble and granite. Granite is much harder and can withstand centuries of weathering. Granite does not need to be covered or sealed outside but sealing it will provide stain resistance and will further enhance its weather-resistant properties. Marbles, as well as manufactured or cast stone, are softer and will degrade over time, especially in cold climates where water enters microscopic cracks and expands upon freezing, eroding the rock. Put your granite fountain outside and be confident!
