How are blacksmithing bonuses determined?

Currently, I have my blacksmithing at the highest level, as well as my sagecrafting. I've been socketing epic gems into weapons during the crafting process for a few levels now. I am a little confused as to how blacksmithing bonuses are determined, especially since my crafting stats are maxed. Here's what my screen looks like:

As you can see, every weapon is eligible for a bonus except Chakrams and Scepters. What part of the blacksmithing process determines whether or not a weapon is eligible for a bonus?

Chakrams and Sceptres can only have Utility gems added to them via blacksmithing, instead of Weapon gems like other weapons.

Bug or feature? That, I don't know.

The bonus is always a gem, but you need to be high enough in the blacksmithing tree. I think it's level 9?

Chakrams only use Utility gems (same for Staffs apparently) - I found a green chakram once around level 23 or so which had two utility sockets.

breakdown is this (incomplete list but this is what i've found):

• bonus for gloves / boots / pants / hats - utility gem or armor gem

• bonus for longswords / hammers / greatswords - utility gem or weapon gem

• bonus for chakrams / staffs - utility gem

if you're seriously considering blacksmithing would also recommend sagecrafting to level 9, which lets you craft epic gems.