The Jewel Trade
After calming Divine Beast Vah Rudania, you can unlock this sidequest. A Gerudo by the name of Ramella will be wandering through Goron City. She is a traveling merchant looking to stock up on her inventory of gems. As any merchant would, she is always happy to buy gems off of other travelers willing to let go of their collections.
As a first request, Ramella will ask you to bring her 10 pieces of Amber. If you already have them in your bag, feel free to hand them over. If not, start looking for black ore rocks scattered around Hyrule's mountains and break them open to collect what's inside.
For your troubles, Ramella will give you 500 Rupees, more than what you would have gotten if you had sold the 10 pieces of Amber to a standard shop. From now on, whenever you want, you can sell her any kind of gem in sets of 10 for more than their market value.