For the coal actually it is more like a 20% increase as it only regards regular rocks that drop coal and not the other ones so you really shouldn't expect that much of an increase for an overall SC trip.
I think the geode one works like the coal but I'm not sure the spawning algorithm, so instead of having double drops them you get a higher chance.
A reasoning thing for this is that the gemologist profession which increases gems also increases geodes and having basically 2 of the same skill is not a great design choice, especially when you can only have those 2 at a given time for mining.