Ruby Dropped by certain villagers carrying chainsaws Sell: 10000 PTAS Note: you can kill 2 chainsaw villagers at the very beginning by going to the exit then entering the building where he should normally spawn. Sell: 2500 PTAS Emerald Found in many places on the island. Sell: 2000 PTAS Velvet Blue Found in many places in the castle.
Spinel Found in many places in the village and in several places in the castle. This includes treasure not shown on the map. The Merchant sells a treasure map for each region marking where the most valuable pieces are to be found. Some treasures like the Beerstein or the Elegant Mask have places where small gems can be inserted, raising its value.
These can be collected and sold to the Merchant. As you go, you will encounter gems and treasures.