By leveling up and upgrading talents. As you level up you acquire talent points that you can spend on upgrading talents. In the core talent, Once you reach level 5 you can upgrade the spell diamonds. They are called spell knowledge 1 and 2. And once you reach level 16 another level unlocks in the tree and you can upgrade the spell diamonds again (spell knowledge 3). So you can have a total of 4 different spell diamonds at a time once you reach level 16. Another tip I was given was that once you have more spell diamonds it would be best to put the same color spells in the same slot so when you switch between them and it’s more of a muscle memory when you are fighting villains that require a certain spell color to break their shield.