Create an actor subclass for the jewels. In the world, have two for loops iterating through all possible positions and at every position, create a random jewel.
If I was to try to generate a board with the code, where would I need to start?
As with any scenario, you start in your World subclass constructor. Your world can be either with pixel-sized cells or use a grid of large-sized cells. You would then add code to build the world and add Actor subclasses as needed. Only one subclass of Actor is needed for all gems -- or, rather, the main playing tiles. A field of int or String value, or just its image (which can be given randomly), can be used to identify which type piece the tile is.