To get to Dog Isle take the row boat from the bottom of Vine Isle.
Go talk to Isaac over on Dog Isle and see if he has chores for you to do, then comment on his guitar. It turns out that his father was a master crafter and he hopes be the master crafter that his father was. Isaac is a gifted crafter and tells you he is working on a new design to honor his father's artistry. However he doesnt have all the materials to make the inlay stand out. Finally something you can help him with!
First he needs 20 rubies and a chunk of obsidian and some lapis lazuli he gives you 2000 for traveling expenses. Obsidian you can obtain from Lava or buy at a rock shop along with rubies.
For the Lapis Lazuli, travel to Jungle Isle to the 3rd Jungle temple up on the northern part of the island. Use your binoculars to find it. It's located right above the fire flame on the right side.
Bring everything back to Isaac. Now he can finish the design!
Reward: 45,000 and 125 QP