TL/DR: the mining changes seem to have severely affected its usefulness.
So I spent a couple of game days this test period mining to see the effect the gem drop changes had on my gameplay. First I'd like to point out that mining is a relatively energy-intense activity compared to the other farming skills (nibbling/harvesting/croppery/etc...) but this has traditionally been offset by profit and earthshakers to negate the energy expenditures. Using earthshakers to negate the energy cost results in long farming periods (I normally do one to two hour runs) since it's more efficient to do it all at once since simply travelling to and from Ilmenskie takes a lot of time (what other reason is there to go so far away from the subway?). People consider mining to be a tedious process, but I don't find it to be any worse than the other skills, and certainly less tedious than squeezing those annoying chickens!
I found myself wanting to spend more time with my other skills than I have previously. I don't have any numbers, but the reduction in gem drops seems to have cut out a lot of the profit I made during these long mining sessions. Given, I don't have max engineering, tinkering, or Alchemy, but I do have admixing to make powders. The profit from Shrine Powder is ridiculously poor imo. After mining, crushing, compounding, and powdering, the amount of energy expended is liable to be in the realm of two to three thousand (without earthshakers/humababas/no-no) and sells for about 2500 currants. The gems have really been the "profit" in the mining endeavor for me in the past. With the changes to gem drops I spent hours mining ore and only netted a few thousand currants worth of gems. It seemed like a profound waste of time when all was said and done (3 hours, and not even 16 gems!). Especially considering I can harvest trees and animals much more readily, have a much greater chance of having random items drop (drinks, musicblocks, harvesting bonuses), rapidly craft those items into higher valued items (for much less energy) and sell them for more money. I don't feel that mining is worth my time in its current incarnation.
I like the idea of having a greater chance of gems dropping at the start of a mining expedition, but I am definitely against having the chance of gems dropping getting increasingly lower with every occurrence. I propose a floor on the drop rate of gems so that those of us who enjoy long mining sessions are not punished so harshly for doing so. I don't have a problem with lower the drop rates, or the amount of profit from mining (it was rather easy to rack up currants from mining), but I feel that this change takes it too far.