Runestones

Runestones let you change how spells look and function. The five types of runestones are Crimson, Indigo, Obsidian, Golden, and Alabaster. Runestones have at least six ranks, the higher the rank the more powerful the effect it adds to the skill. Runestones add depth to character customization, making it so that instead of only choosing six from 20 skills, you are really choosing from 6 times that many.

Examples

Whirlwind is an iconic Diablo II skill, wherein the barbarian spins furiously and damages any enemies in his path.

Crimson - Shoots bits of rock and gravel in every direction, dealing area effect damage.
Indigo - Barbarian spawns smaller whirlwinds which deal direct damage.
Obsidian - Provides the barbarian with immunity to immobilizing effects.
Golden - Reduced Fury cost.
Alabaster - Critical strikes confuse enemies, forcing them to attack their allies for a short time.

Sweeping Wind is a combo skill that lashes out against all enemies surrounding the monk, creating a vortex of damaging wind that increases in damage and radius across three stages.

Crimson - A flurry of blades increases Sweeping Wind's damage.
Indigo - Expands the area of effect and strikes enemies with additional fire-based attacks.
Obsidian - A persistent whirlwind surrounds the monk, dealing damage to any foes that come into contact.
Golden - Increases Spirit regeneration.
Alabaster - A powerful sandstorm billows with the monk's attacks, damaging enemies and knocking them back.

Ray of Frost is a wizard spell that damages and freezes enemies caught in the beam, slowing their movement and attack speed.

Crimson - Beam increases in damage the longer it remains in use, to a cap.
Indigo - Wizard is surrounded by a swirling vortex of sleet, damaging any foes that come into contact.
Obsidian - Decreases beam damage, but increases reduction in target movement and attack speed.
Golden - Arcane Power cost reduced.
Alabaster - Enemies slain by Ray of Frost have a chance to leave an area of ice on the ground where they die, damaging other enemies if they move through it.

Cluster Arrow is a powerful demon hunter tool, as it fires a cluster bomb which explodes on impact, releasing smaller bombs which explode shortly thereafter.

Crimson - Increases the damage of the explosion at the point of impact.
Indigo - Fires enemy-seeking missiles instead of grenades.
Obsidian - The Cluster Arrow is launched high into the air and rains down grenades.
Golden - Instead of grenades, the skill spawns shadow creatures that attack nearby enemies and leech life to the demon hunter.
Alabaster - Adds a stun effect to the grenade explosions.

Acid Cloud is a witch doctor skill that summons an acid-spewing spirit, raining caustic, skin-dissolving death on the unfortunate enemies below.

Crimson - Causes immediate damage instead of a damage over time effect.
Indigo - Increases area-of-effect radius.
Obsidian - Spawns slimes that attack nearby enemies.
Golden - Afflicts enemies with a disease effect that deals damage over time.
Alabaster - The witch doctor belches forth a directional pool of acid.

Picking your runestones allows you to tailor your abilities and cover your weaknesses or enhance your strengths. They allow you to squeeze more utility and damage from your skills.

Runestones