As a geologist I find a lot of things about mining (my favorite Haven activity) to be rather disheartening.
Many of them just can't be changed, such as ensuring that proper rock types are next to each other or gems being found in non gem bearing rocks, and I can accept that.
However, something that constantly bothers me is rock hardness.
I would like to see the Mohs Scale for Mineral Hardness implemented in the game. https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
Basically, limestone (made of Calcite, the defining 3 on the scale) should NEVER be harder than say quartz (the defining 7 on the scale) on a given mine level. Sure, Limestone on L5 should be harder than Quartz on L1, but on any given level, even with hardness nodes.
This would be more challenging when we get to the igneous rocks in game: porphyry, basalt and granite. However we can use the average % of these rocks to determine what they should have. This would make granite slightly less than 6 (feldspar), porphyryr can be assumed to be between feldspar and quartz making it ~6.5, and basalt slightly less than 7 (where the mineral olivine lies)
We would have to do something similar with the metamorphic rocks: schist, marble, gneiss.
This is only a starting point idea, and not set in stone