Rosella:
Of course, you probably already know that water is the symbol of our mother Eva. Therefore, unlike other races, we elves receive the divine protection of mother Eva in the water. You need not go far to find the pure aquamarine, that symbolizes water. It can be found in the Elven Forest. Go to Thalia who tends the Mother Tree at Lake Iris. Give this fruit to her as a give, and ask her about the jewel. She will gladly help you.
You can reach Lake Iris if you go straight south of the Elven Village. Thalia will be at the western shore of the lake. Well then, good luck.