Bluegleam has an important role in Banuk's culture. It resembles a crystal and it grows on machines' wrecks when the climate is right. It serves as a currency.
If you want to buy a legendary equipment offered by Banuk merchants, you need to get a certain amount of Bluegleam. These crystals can be acquired in two ways.
The first one is to complete quests. You get Bluegleam as your payment for helping Banuk tribe. This way is rather easy because you don't have to complete additional activities that aren't connected to a given quest.
The number of quests is limited which means that if you want to get more crystals then you will have to search for them. This is the second way of getting Bluegleam. It can be found throughout the entire land. They are a characteristic grow that can be found on destroyed machines.
Of course, it doesn't grow on every single one of them. You may have a hard time finding them. Buy a map from one of the merchants to make things easier. Those maps can show you areas where Bluegleam can be found.