1finding one
You do not craft this. It grows in place of the occasional Crystal Shard in the Underground Hallow, so the fastest way to get one is the same trick people use to farm shards: lay long rows of Pearlstone across a lit hallowed cavern, walk away, and harvest whatever has sprouted when you come back. Every crystal you break is another roll.
Spotting it takes some care, because the minimap still labels it as an ordinary shard. Go by the object itself, which pulses between pink and blue while plain shards hold one color, or carry a Metal Detector and let the readout tell you.
using it
Queen Slime only answers inside the Hallow, above ground or below, and the time of day does not matter. Worlds made before she existed still generate these, so there is no reason to start a fresh save. Bring several if you plan to fight her more than once, and note that a second one does nothing while she is still alive.