Is half a syringe of Restylane enough to get noticeable results under the eyes?

It's impossible to answer your specific question about the amount of Restylane needed for your under eyes without looking at them.  Most people require at least one syringe total (one half per side) to see a significant result, but this varies widely. With dark skin, you may be experiencing an "accordion effect" where the skin bunches and looks darker because of micro shadows.  Often fillers can help but you may still need to use concealer or BB cream for a complete improvement. Caution: the importance of the amount of filler used is secondary to the skill of the injector.

The under eye area can be treated with fillers, lasers, microneedling/PRP for improvement. I would suggest posting photos and consulting with an experienced board certified dermatologist for evaluation. Best, Dr. Emer

Usually half syringe is enough for under eyes since need to be conservative to avoid overly done and swelling effect. Every one is different. See video below and see an oculoplastic specialist.