The capital of a country is built on a series of stone hills that gradually lead to the ocean. The stone is noted for both the strength of castles made from it and its unique blue-green coloring. This city is surrounded by the ocean on one side, and steep mountains (which are responsible for the area's long rainy season) on the other.
What would be the best stone to use for these hills?
The first stone type is sedimentary. I think this one is unlikely, as sedimentary rocks don't usually exist in a blue-green color, aren't the best at withstanding erosion from constant rainfall, and generally aren't very good for building castles. Coquina, on the other hand, is one example of a sedimentary rock which would be good for building fortifications.
Next are igneous rocks. The color could come from underground mineral deposits, and the hills themselves could have been made from gradual lava flows. Igneus rocks can be very strong, withstanding both erosion and attack. Granite is a great example.
Metamorphic rocks are last. They can come in many different colors, with blue and green already being fairly common. While generally not as hard as igneus rocks, they could start as igneus rocks and morph over time, eventually being uplifted to the surface.
So, in summary, my question is this: What rock can I use that is strong, contains a mineral that makes it blue-green in color, and can form in a series of hills?