The Dark Crystal is a dark crystal orb and an object in the Dragon Ball series. It is an artifact in which Garlic was trapped when he forcefully trid to take the Guardian position and take over Earth before being defeated and sealed away by the Nameless Namekian (before Kami and King Piccolo came into being upon splitting).
The crystal itself is an opaque and pitch black star dodecahedron (3-dimensional Star-of-David shape).
It is kept in Kami's Lookout.
Garlic was sealed in the Dark Crystal in a similar fashion that Kami and Mr. Popo were captured and contained in clear bottles by his son Garlic Jr. One may infer that Garlic was kept in that crystal for the remainder of his life. It is possible that the crystal is somehow connected to the Dead Zone, thus all who are sealed in it are trapped within the Dead Zone.