Medusa Scales
Medusa Scales are obtained by using a dagger or bladed weapon to harvest from Medusa while she is dead OR alive. There are 2 different hues of Medusa Scales:
Invulnerability Blue while she's alive, 
Darker blue from her corpse.
The brighter scales are much more desirable as they yield 100% effective Gorgon Lenses, whereas the darker lenses are only 90% effective.