The biggest stones give 10% if they didn't change anything. Block is balanced arround the usage of a shield and far less beneficial with weapons.

Have not seen any gems nearly that high, although I'm not THAT far in yet. 6% is the best I've seen also.
Block chance and block efficiency are two totally different things though, and it being balanced around shields has nothing to do with what he's talking about. While shields DO add base block efficiency as a flat %, efficiency gems and passives add it as a % of your current efficiency, which as the OP stated makes no sense.

Even with a shield, my block efficiency is quite low with the addition of gems but my block chance is enormous because all heavy armor as well as passives and the shield add FLAT block chance. The only block efficiency passives, as well as the gems, give a % increase to your CURRENT block efficiency.

Choosing to play a more supportive role like tank/paladin style of gameplay feels like it's being punished, tbh, and restricting the variety of builds players can choose is never a good idea.

At least one or the other, passives or gems, needs to add flat efficiency to blocks imo.