From https://game.nanoo.so/darksword/forum/455641?code=5b709c13b17d29f4369768de22e5ed68 (Official Guides>Equipment (Types, Tiers Grades and more))

 

All equipment be strengthened through upgrades. ◼︎ Tap on Equipment to upgrade ◼︎ Gold is required to upgrade ◼︎ Upgrades of up to 100 levels for Legendary equipment, 80 for Epic, 60 for Rare, 40 for Magic and 20 for Normal are possible ...

 

Equipment that have reached their limits can break limit and be enhanced to a higher level.

5 Stones matching the weapon attributes are required to break limits

 

-Weapons, helms and armors with Physical attribute need 5 Iron Stones.

-Weapons with Fire attribute need 5 Red Stones

-Weapons with Ice attribute need 5 Blue Stones

-Weapons with Lightning attribute need 5 Yellow Stones

-Weapons with Poison attribute need 5 Green Stones

-Weapons with Wind attribute need 5 Wind Stones

-Weapons with Dark attribute need 5 Dark Stones

-Necklaces and rings need 5 Magic Stones ◼︎

 

Increase strengthening level by 5 upon breaking the limit ◼︎

Breaking limits are infinite as long as you possess the Stones ◼︎

Stones are dropped at a low rate upon eliminating monsters (They can be purchased at a low rate on Extra Bonus)