The Rainbow Caves has all 5 colors of quartz including pulsar, solar, blazar, magnetar, and quasar. These are essential resources for end game crafting and are used in things like:
Fusion Energy Cells
Drone Stations
Nuclear Fusion Generators
Portal Generators
Teleporters
Extraction Platforms
T6 Backpacks
T4 Jetpacks
Warden Key Detector
Those glowing crystals allow you to access some of the best stuff! So finding these caves are a key part of your progression and this video shows you where you can find them. But I'll also cover where else you can get quartz either by crafting them or doing portal runs.