Pirou is looking for 55 Rushrooms and in exchange, he will give Link a Diamond. This quest is not recommended to be done all at once. As Link goes on his adventure, he should slowly find Rushrooms, as they commonly grow on the sides of cliffs.
After Link has gathered 55 of them, he can turn them into Pirou. Pirou won't actually give you the Diamond directly, but instead, he will tell Link that it is in a treasure chest right behind the stable. This will complete the quest, but Link can sell Pirou 55 more rushrooms at anytime and he will pay double the going rate for them.
Tip: If you have done the Slated for Upgrades Side Quest and have the Sheikah 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.
Speak with Pirou to start the quest. Give Pirou 55 Rushrooms. Don't forget to open the chest behind the stable for the Diamond.
|Pirou at the Gerudo Canyon Stable claims that he can't live without rushrooms, but because they grow on cliffs, he's unable to pick them himself. He's offered to trade a diamond for 55 rushrooms.|
|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.