MArcomage

Why would I spend 11 gems and an Uncommon slot to exchange 4 stock with a very situational condition?

Because you're going for a resource win.

You're spending 11 resources to gain 12 resources, a net change of 1. This card is a drop in the bucket for a resource win. Your opponent feels more change than you do.

What this is, as I see it, is a card that can be easily added to Brigand+Destruction decks that may not otherwise using gems to a great extent, all in hopes of keeping your opponent's stock next to nothing. It doesn't really stand out at all, though. Aside from the restrictive condition, it's not very noteworthy.

exactly my point. Increase the stock gain or decrease the cost.

EDIT: I see u increased the stock exchange to 5, but that still leaves a net gain of +4 resources. If I was going for a resource victory, I would expect at least +10 stock net gain. Maybe decrease the stock to 5 gems

Or make it durable and rare :) with some addition, for example that it can't decrease stock below 10.

even if it was a rare durable I'd still not take it like this. I could easily use Bank and go to economic victory in 5 rounds