Green Strength
With a little luck, this will add a new bonus to one of your items (max. 4 bonuses). Can only be used on weapons and armour up to level 40.
Green Magic
Removes the bonuses from one of your items and adds new ones. Can only be used on weapons and armour up to level 40.
Green Magic & Green Strength can drop from metin stones from lv.5 to lv.40, additionally you can drop normal ones too check here where.
Level > Name >Spawn areas
5 > Metin of Sorrow > Joan, Pyungmoo, Yongan
10 > Metin of Combat > Joan, Pyungmoo, Yongan
15 > Metin of Battle > Joan, Pyungmoo, Yongan
20 > Metin of Greed > Joan, Pyungmoo, Yongan
25 > Metin of Black > Bakra, Bokjung, Yayang, Joan, Pyungmoo, Yongan
30 > Metin of Darkness > Bakra, Bokjung, Yayang
35 > Metin of Jealousy > Bakra, Bokjung, Yayang
40 > Metin of Soul > Yongbi Desert