After a little bit of testing, I believe that the “More gemstones” (i.e. Sturdy Mining Helmet, Excavator Gloves, etc.) effect on mining gears is completely broken, or I just got weird rolls for RNG.
Exact mining gear for reference:
Master Pickaxe; Sturdy Mining Helmet; Excavator Moustache; Excavator Gloves; Deep Dweller Shirt; Deep Dweller Pants; Excavator Boots.
2 of these gears have the effect “More gemstones”.
After running a couple of mines, I noticed that I would still get a minimum of 3 gemstones (happened quite a few times), would never get more than 7 gemstones (the maximum of such pickaxe) and it seemed to have the same rate compared to just not wearing the gear.
Based on the description of items with the “More gemstones” effect, there should be an increase in the maximum (hence the wording “extra gemstones”), or it’s meant to increase the rates of getting more gemstones, which doesn’t seem to happen (unless I’m unlucky, I guess).
The description for reference: