The Celestial Crystal Cluster is a block added by Astral Sorcery. It is used to acquire Celestial Crystals as well as Stardust once it has fully grown. If broken prematurely it will only drop Stardust. The rate at which growth occurs can be increased by shining starlight from a Collector Crystal or Celestial Collector Crystal on it. When broken, 1 Stardust drops 100% of the time and a second Stardust drops 50% normally, or 100% of the time if broken with a pickaxe enchanted with Fortune; the enchantment level does not matter. If the Celestial Crystal Cluster is broken on a block of Starmetal Ore, there is a 33.33% chance for an additional Celestial Crystal to drop; the Starmetal Ore can also randomly be transformed back into Iron Ore while the Cluster resides on it.
It can be crafted by dropping a Rock Crystal and Stardust into Liquid Starlight. After a few seconds all three will be removed and replaced with the Celestial Crystal Cluster.
Astral Tome entry
The Stardust appears to be far more useful to experiment with than the raw Starmetal Ore. The little grains occasionally flash with light and sparkle while flowing like sand against an ocean tide.
As you've not found any direct uses from the Ancients, experimentation by rubbing it on various celestial items and devices has revealed it readily bonds to the surface of Rock Crystals in a thin, but rough, coating.
This is not helpful in itself, and refused to be removed from the crystal without grinding.
Since removal of the rough coating would leave you at square one, changing how the dust collects on the crystal surface may be a better approach. Placing the Stardust into Liquid Starlight, and subsequently tossing in a crystal, seems to result in more even application of the dust, as well as a promoting growth of the crystal into a stable cluster that seems hungry for Starlight.
Leaving this cluster to grow with Starlight shining on it may create something much more useful, and these clusters seem to interact symbiotically with Starmetal Ore itself.