forging items for 0 gems

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Previous threads have indicated that item forging costs a minimum of at least 1 gem, no matter what bonuses are in play.
Well I dont have the best forgers around, but in an SP game, a Jade Sorceress of mine forged Rainbow Armor for 0 gems. I was pretty surprised, checked a few times.
The deal: Forge of the Ancients is up, she has Hammer of the Forge Lord equipped, and she is in a province with a 20 Construction bonus (Ancient Forge I think the site is called). Neither Air gems nor Nature gems are subtraced when she is given the order to forge.
I have not tested this systematically, so feel free to throw rocks at me.
Apologizes if this is old news, but I did search and turned up info to the contrary.