A very nice addition to your mermaid collection could be the Neptunes Daughter Mermaid Ring. It’s made from 14kt gold and has a nice beautiful diamond in the navel of the ring. It’s measurements are 7/16 inches wide and 5/16 inches high.
The makers refer to the ring as having a gentle sadness, and I guess it’s all about how you perceive it. It seems to me like the mermaid is protecting something that is valuable to her, and if you give this ring as a gift, you could be saying you are protecting them in the same way.

The price is steep, but worth every penny!
The price is steep, but if you want the best sometimes you have to pay for it. It goes for $1230. Their jewelry is made to order, so the exact shipping time will vary.
You of course can order the ring in different standard sizes so you, or the loved one you will be giving this ring as a gift for will get the size they need.