Loadstone

From Tales From the Yawning Portal, page 228.

Description
''Wondrous item, rare. 1 lbs.''

This stone is a large gem worth 150 gp.

Curse. The stone is cursed, but its magical nature is hidden; Detect Magic doesn't detect it. An Identify spell reveals the stone's true nature. If you use the Dash or Disengage action while the stone is on your person, its curse activates. Until the curse is broken with Remove Curse or similar magic, your speed is reduced by 5 feet, and your maximum load and maximum lift capacities are halved. You also become unwilling to part with the stone.