No, on every count. Firstly: no coal. Coal is the fossils of trees, primarily of the Carboniferous period. Life is required for coal, and active geological processes. Secondly: Pressure is a force exerted inwards, toward the centre of a planet, due to the weight of all the mass above it. Geological processes do force material to the surface, and a diamond planet will likely have none anyway. Diamonds of any size are not known for their internal activity.
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