The Nightstone Shard is a small rock about the size of a peach. It appeared first to Z in Old Xoblob's Shop as a chunk of coal. After purchasing it, it was discovered that Sir Tiberious, a paladin of Helm, saw the stone as a sparkling red gem.
The shard has since been reunited with the rest of the Nightstone, in Is There a Plan Here? [Session 37]. When the two pieces reunited, the The Nightstone, now whole, shrunk down to fit into one's palm.