Each of the runestone types - Crimson, Indigo, Obsidian, Golden, and Alabaster - grant a different effect, which also depends on the ”different ranks of power”.
There are over 500 possible combinations to choose from in the game thanks to the inclusion of runestones which amp up our skills in varying ways depending on their type and rank. Blizzard has lifted the shroud of mystery from the runestones.
”There are five runestone types: Crimson, Indigo, Obsidian, Golden, and Alabaster. Each runestone affects skills differently,” reads the official Diablo III website. ”For example, adding a Crimson Runestone to a skill may enhance its damage, while adding an Alabaster Runestone might add a stun effect.”
”In addition to their color, runestones also come in different ranks of power; the higher a runestone’s rank, the more dramatic its effect on the skill. Experiment, and discover which skill and runestone combination best suits your personal playstyle.”
You assign a runestone to a skill through the Skills screen say Blizzard, while a tooltip explains what effect it will have. Damage could increase, so could area of effect radius or perhaps an ability will become a lot more precise. Check out the Diablo III runestones.
Diablo III releases exclusively on PC at an undisclosed date.