Would it be possible to implement magical diamonds as an armor percentage increasing gem. In theory it would only be slotted into chest armor only and would not mix with other gems. As such it would have a higher base percentage, lets say 5-10% increase vs all weapon types per gem level.
These gems could be implemented as a very rare gem to gain from any city, which you are more likely to receive with a high rank or a findable gemstone from certain varieties of monsters or any profession that yields gems, though at a lower ratio then the gems currently given from those.
I figure magic diamonds of this variety would allow armor to scale somewhat along with gem weapons and possibly give people a bit of leeway in their choices in armor.