Maker - Alchemy Skill FAQ and Walkthrough

A weapon is an item nontheless. And actually, you can only put one of each kind, regardless of the grade. So only 1 diamond, making it a maximum 3 attack addition.

However, you can try your luck with a black crystal, that adds more stats randomly, included attack on weapons, stacking with any diamonds you may be using.

I have a good example of this. I forged an Asianic bow the other day, and used 1 low grade diamond (1 ATT). However, when the item came out, it had +4 attack above average. so, I got +1 from the jewel and +3 random increase. Imagine if I had been using a +3 ATT diamond...