Rushroom Rush!
Requirements
Talk to Pirou at the Gerudo Canyon Stable to start the quest.
Walkthrough
Turn in 55 Rushrooms to get a diamond. He will not give it to you directly; it will be in a chest behind the stables. Rushrooms are everywhere; they are the purple mushrooms that grow on cliffs.
Tip: If you have done the Slated for Upgrades Side Quest and have Sensor+, take a picture of a Rushroom. You can then set it as your Sensor+ target (#179 in the compendium) for help in finding more of them.
Bonus Tip: If you do not already have a picture of a Rushroom, and if you have done the Sunshroom Sensing Side Quest as well, you can talk to Symin at the Hateno Ancient Tech Lab and purchase materials pictures for your compendium (at 100 rupees each, getting a random picture each time, can buy in bulk if desired) until you have a picture of a Rushroom. You can then set it as your Sensor+ target for help in finding one.
Quest Description
Completed Quest
You traded 55 rushrooms to Pirou at the Gerudo Canyon Stable for a diamond.
But it seems he could still use more.
If you bring them, he'll pay twice the market value for another 55 rushrooms.