This item is an experience material.
To strengthen your girl by using this gem, It will cost :
Cost = n x ★ x 【 400 + 3 x ( level -1)】 CC
n = Number of Forest gems ++
Example : Mami 5 ★ lvl 86
Using 9 Forest Gems ---> Cost = 9 x 5 x 【 400 + 3 x ( level -1)】= 29475 CC
It gives 2500 exp , an extra x1.5 exp bonus if the girl is of type 
.