Six elements can be put into S-grade weapons and armor: a weapon then hits harder, an armor piece resists the opposite element. This page explains the stones and crystals, the limits of each item, the chance of success and where the stones come from.
Six elements in three pairs
The stones are 
Fire Stone and 
Water Stone, 
Earth Stone and 
Wind Stone, 
Dark Stone and 
Divine Stone. Each pair is a pair of opposites: fire against water, earth against wind, dark against holy. Opposite attributes cannot be put into the same item, and a stone can be exchanged for its opposite at the Attribute Masters: Yin in Aden and Yang in Rune.
Weapons
- A weapon holds one element. The first successful stone gives 20 points, every next one 5.
- Stones work up to attribute level 3, which is 150 points. Levels 4 to 6, up to 300 points, are reached only with crystals such as
Fire Crystal. - 150 points add roughly 70% to damage in both PvE and PvP when the target has no resistance to that element.
- Fighters may choose any element. Mages take the element of their main spells, archers the one their skills are enchanted for.
Armor
- A stone adds 6 points to a piece and gives resistance to the opposite element: Fire Stone in armor protects from water, Water Stone from fire, and so on.
- One piece takes up to three different elements, 60 points each with stones and 120 with crystals.
- Resistance is subtracted from the attacker's attribute. With 120 resistance against a weapon of 150 you are hit as if the weapon had 30.
Chance and risk
A stone takes hold about every second time. A failure destroys only the stone: the item is safe and the points already gained stay. Filling a weapon to 150 therefore costs about sixty stones.
Where the stones come from
Stones drop from monsters of the high-level hunting grounds; the page of each stone lists who drops it and with what chance. Many players farm fire stones on the Isle of Prayer near Heine, where a party of more than two players provokes dangerous guards, and earth, wind and holy stones in the Chromatic Highlands and on Hellbound.
Buffs count too: a resistance buff or a self-buff of an element adds to the same numbers, so a mage with 130 points and a buff hits like one with 150.