Luminous Stone Gathering
After calming Divine Beast Vah Ruta, a Zora named Ledo will offer up this sidequest to you. During the day, Ledo is working on repairs around the central area in Zora's Domain, so he should be pretty easy to find. Talk to him and he will offer you two Diamonds in exchange for ten Luminous Stones.
Luminous Stones can be found from those glowing blue-green stones that are scattered around Hyrule. Give Ledo ten to complete the sidequest and receive your promised two Diamonds.
In the future, Ledo will give you one Diamond for another set of ten Luminous Stones. If you are in desperate need of specifically Diamonds, then make the trade. But if you want to sell your precious stones for the maximum number of Rupees, don't take up the offer. A Diamond will sell for 500 Rupees, but ten Luminous Stones will give you 700 Rupees (70 Rupees each).