Materials from the same gathering spot can be exchanged for rarer
materials at the Exchange. All materials can also be exchanged for
Six-Flower Orbs.
Once you have unlocked every map, the Treasures tab of the Exchange
lets you exchange all of your materials for Gold or Six-Flower Orbs
in one go.
When exchanging Malachite for Stifim, ticking “Use all lower-tier
materials for the exchange” means Yugresta Grass is used as exchange
material too.
Tapping a material in the Items list opens its detail screen.
There you can sell materials in lots of 100.