The Cosmos Crystal, or Cosmos Seed, was the crystal of the Galaxy Cauldron. Protected by Guardian Cosmos, it released the ultimate universal Lambda Power when awakened, and that power was strong enough to recreate and rebirth an entire galaxy worth of Sailor Crystals. It was never shown in the manga, but its effects were easy to see.
In Act 60, the Cosmos Crystal was activated through Sailor Moon, and all of the Sailor Crystals trapped in the Cauldron were reborn and their power attacked Chaos until it was almost completely destroyed. Afterwards, the Lambda Power sent all of the Crystals back to their planets, where they resumed the forms they had before they were destroyed by Sailor Galaxia, as explained by Sailor Cosmos.
At one point, Guardian Cosmos made several ambiguous statements about how a Sailor Crystal much like Sailor Moon's had been born from her and was cast off, seeking to create a new history for the stars and then return when it was finished. Queen Serenity was visible in the background as Guardian Cosmos spoke, but the meaning was unclear.