Hello,
to start this off...no, this is not one of those posts where people complain about drop rate.
However I would like to point out that after grinding for a few days this occour to me, if @Daneos thinks it's not complicated to do I would like to suggest that when upgrade stones drop just like devil coins they should stand out over the other drops because personally i find it hard to see them and i noticed when devil coins dorp the icon goes on top of every other armor / weapon whatever that drops.
If this was already suggested I do apologize in advance.