Blizzard wants players to be able to have the option to highly customize their characters in Diablo III. In order to accomplish this lofty goal, Blizzard has introduced Runestones. Runestones are used in order to augment your skills. Each class has 20+ Skills and players are allowed access to seven of those Skills at any one time.
There are five types of Runestones: crimson, indigo, obsidian, golden, and alabaster. In addition to the different types of colors, Runestones also come in different ranks of power. The higher the rank, the better effect on the skills it is attached to. Each of the Runestone’s effects vary depending on what skill you actually attach it to. For instance, the crimson Runestone will enhance a barbarian’s Whirlwhind Attack to cause rocks to fly in every direction, dealing area of effect damage. The same crimson Runestone will cause the witch doctor’s Acid Cloud spell to cause immediate damage instead of its original damage over time effect.
To see videos on how Runestones alter your Skills, head on over to the official Diablo III website.