I don't know if it works on Tanzanite/Pear/Diamond as didn't got to that stage yet but if you shift-LMB-click amethyst leaves 2 will be dropped per 1 broken. Yep this is in 1.9.8.

Just realised that this is working on Tanzanite. And it's interesting what would be there if it was working on Diamond...

Found some interesting details.
This dupe works on all 4 trees. And with any shears (tested vanilla, thermal foundation, extra utilities precision shears (both LMB and Shift-RMB methods), RC steel shears and RA omniwrench), I can say more: the leaves duplicate even without Shift. And I think I caught the reason: they can be harvested by hand, but the CalculatorLeaves class is implementing IShearable so when I harvest they with shears they drop 1 block as expected + 1 block as harvested with shears = 2 blocks from 1. Please fix.

Fixed in dev