Crystal Clusters are items that can be discovered during the Research process. The type of crystal doesn't seem to change anything, but the crystal will gradually produce Vis for the nearest Aura Node. It can also over-boost an Aura Node to about 10% over its maximum capacity, but doing so will cause a small amount of flux to be produced. One crystal produces 1 Vis every 5 minutes for the nearest Aura Node (you can see how much vis the Node can store with the Goggles of Revealing).
Crystal Clusters can be used with Silverwood Saplings to quickly chain-link Aura Nodes.
Here's a video that shows you how Crystal Clusters and Crystal Capacitors work:
"You have found a way to fuse the crystal shards into larger, more stable, crystalline clusters. These crystals thrum with magical energy and are quite pleasing to the eye. These crystals seem to be able to slowly replenish nearby aura nodes and even boost their vis levels above their normal baseline levels to a limited degree. Of course that will probably lead to Flux build-up, but the benefits may outweigh the negative side-effects "
Research available after Basic Flux Research.