After Enter the Dark quest Hitomi goes to some rock formation to climbing (see Large Doline). She will find there some Crystals and want to help and give her some more.
To do this you must go to Cave ( use Grappling Hook she give you) and collect Crystals. Yo will need 10 Argonite and 10 Calcite.
Crystals are collected exacly in the way that wood, so you need use Axe and "Right Click".