Jewels can be found in Darude map, Darude is outside the main castle, when you leave you can see him, he is the blonde guy, speak to him go to the map and select the mines and buy,

*mines don't always sell the jewel

Price: 2000 gold

The jewel is important for level up Sandra's relationship.

Also sold by a clerk located in the bottom left building at Thremten (Map no. 9)