You can earn a Skill Stone every time your Hero Mastery increases.
Skill Stones are used to enhance heroes’ skills.
Each hero has 5 skills, which can all be enhanced via Skill Enhancement. Also, the greater number of enhanced skills your hero has, the greater the number of Skill Stones you will need to continue enhancing your skills.
When you select your hero, you can see their Skill Enhancement options on the right-hand side of the screen and select to take them into the arena. Usually, you can select up to 2 enhanced skills. However, you can select up to 3 enhanced skills if all the skills of a hero are enhanced.