Pretty sure this is a bug. So basically diamond pickaxe is an unique variant of pickaxe, yet it doesn't have the same property of normal pickaxe (which has an attack combo of 1). Instead diamond pickaxe has a combo of 2 which makes it very slow and strictly worse than normal pickaxe. I am pretty sure this is unintentionally? As this is not an issue really with diamond sword vs normal sword, and pretty much all other weapons' unique variants are better than the normal version. So please fix the combo of diamond pickaxe to 1 as well, thanks!