Hmm, didn't even consider that the jewelers/gem SYs might be distant from the docks. Brugle's advice probably qualifies as a golden rule: when importing anything by water, make sure the dockers don't have to travel far.
Another piece of advice. Sometimes, even for the more lucrative commodities, it's simply not worth pursuing a trading opportunity because of the disruptions it will create at the dock. It's hard to tell if that's the case in this situation, but you should always consider how a given industry will affect your dock traffic before you build it, and assume the worst set of conditions (if a raw material can be delivered by water, it will be -- will that squeeze out hull space which is needed for other goods, tie up dock time needed for other purposes, etc...). Managing dock traffic is often a central issue in city design. For gems/lux goods, I agree with Merepatra, it's probably not worth much disruption.