Blizzard has gone into greater detail on Diablo III's runestones, which let players customise the effects - and even appearance - of their skills.
There are five varieties of runestone - crimson, indigo, obsidian, golden, and alabaster.
Each modifies the skill its attached to in different ways - Blizzard's post offers the example of crimson increasing damage while alabaster adds a stun effect.
The effects of each runestone type vary from skill to skill, too. For instance, the crimson runestone changes the damage of Acid Cloud from a sustained to an immediate effect, but adds area of effect damage to Whirlwind.
As there are over 100 skills in the game, that's more than 500 possible combinations.
Additionally, each of the five runestones is available in several different ranks, or levels of strength - meaning you're potentially looking at thousands of results.
Hit the link above for more details.